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THE  ANATOMY  AND  HISTOLOGY  OF  THE  HUMAN 
EYEBALL  IN  THE  NORMAL  STATE 


ANATOMIE  UND  HISTOLOGIE 

DES 

MENSCHLICHEN  AUGAPFELS 

IM  NORMALZUSTANDE 

SEINE  ENTWICKLUNG  UND  SEIN   ALTERN 


Von 

DR.  MAXIMILIAN  SALZMANN 

A.  Professor  dcr  Augenheilku7ide  in  Wien 


MIT    5    FIGUREN    IM    TEXTE    UND    g    TAFELN    IN    LICHTDRUCK 


LEIPZIG  UND  WIEN 

FRANZ    DEUTICKE 

1912 


THE  ANATOMY  AND  HISTOLOGY 

OF 

THE    HUMAN     EYEBALL 

IN  THE  NORMAL  STATE 

ITS  DEVELOPMENT  AND  SENESCENCE 


By 

DR.  MAXIMILIAN  SALZMANN 

Titular  Professor  of  Ophthalmology 
University  of  Vienna 

Authorized    Translation 

By 

DR.  E.  V.  L.  BROWN 

Imiruaor  in  the   Pathology  of  the  Eye 
The  Unifersitf  of  Chicago 


WITH    5   TEXT   FIGURES  AND   9   PLATES  IN  PHOTOCOLLOTYPE 


CHICAGO 

1912 


QMtll 
19IZ 


Published  August   1912 


Composed  and  Printed  By 

The  University  of  Chicago  Press 

Chicaeo.  Illinois.  U.S.A. 


Dedicated  to  My  Revered  Teacher 

THE    Thorough    Student    and    Investigator    of    the 

Normal  and  Pathologic  Anatomy  of  the  Eye 

PROFESSOR  ERNST  FUCHS 

By  the  Author 


PREFACE 

As  in  the  case  with  so  many  other  books,  this  one  has  developed  from 
the  lectures  which  I  have  regularly  given  for  years,  and  I  only  comply 
with  an  oft-expressed  wish  of  my  auditors  when  I  put  the  substance  of 
these  lectures  into  print  from  the  same  point  of  view  as  the  one  by  which 
I  have  allowed  myself  to  be  guided  in  the  lectures. 

This  point  of  view  is  that  a  thorough  knowledge  of  the  normal  anatomy 
and  histology  gives  the  most  certain  basis  for  the  understanding  of 
methods  of  clinical  investigation  and  for  judging  pathologic  changes — 
hence  the  reference  to  ophthalmoscopy,  to  the  physiology  of  accommoda- 
tion, and  to  pathologic  processes  here  and  there. 

I  should  like  to  have  the  book  considered  from  the  point  of  view  that 
it  is  the  eye  specialist  and  not  the  specialist  in  anatomy  who  writes  it. 
For  example,  I  have  not  gone  into  comparative  anatomy  in  a  detailed 
way.  Important  as  this  is  for  the  determination  of  morphologic  ques- 
tions and  much  as  I  am  personally  interested  in  it,  I  hold  it  to  be 
superfluous  for  the  purpose  at  hand  to  enter  into  this  branch  of  learning. 
Naturally,  therefore,  some  details  of  cell-structure,  nerve-endings,  and  the 
like,  which  for  technical  reasons  one  can  study  only  on  animals,  are  treated 
only  briefly. 

Probably  no  one  will  find  fault  with  me  because  I  have  not  encumbered 
the  book  with  the  ballast  of  a  complete  reference  to  the  literature.  Such 
would  stand  in  no  relation  to  the  compass  of  the  text,  which  I  have  made 
as  compact  as  possible.  Extended  references  to  the  literature  are  to  be 
found  in  the  most  cited  articles,  especially  in  the  corresponding  chapters 
of  the  Craefe-Saemisch  Handbuch.  On  the  other  hand,  I  will  not  deny 
that  any  choice  from  our  enormously  swollen  literature  is  an  arbitrary 
one.  Forgiveness  is  herewith  implored  in  advance  from  whomsoever 
feels  slighted  in  this  respect. 

One  cannot  be  original  in  a  subject  which  has  been  so  much  worked 
in  its  entirety  and  in  detail.  Content  and  method  of  expression  must 
necessarily  move  in  the  same  paths  as  those  to  which  the  earlier  works 
have  held. 

At  the  same  time  I  feel  I  may  say  that  I  have  not  simply  copied 
from  others,  but  that  my  descriptions  and  my  drawings  have  been 
made  true  to  nature.  I  have  borrowed  only  one  drawing  (Plate  IX,  6) 
— from  an  embryologic  work;  all  others  are  original  and  with  very  few 


viii  PREFACE 

exceptions  prepared  from  my  own  specimens.  In  so  doing  I  have 
striven  throughout  to  bring  forth  concrete  pictures,  i.e.,  each  drawing 
is  a  true  representation  of  the  preparation  concerned.  Only  the  general 
drawings  of  the  eyeball  of  the  adult  and  of  the  newborn  are  schematic. 
In  Plate  I  (Taf.  I)  some  details  of  the  zonula  and  vitreous  are  drawn 
in  from  other  preparations  and  Plate  III,  2  is  also  a  combination  of 
various  teased  preparations  of  the  same  eye. 

I  express  my  especial  thanks  to  the  publisher  for  his  friendliness  and 
for  the  sacrifice  occasioned  by  the  nature  of  the  reproduction  of  the 
drawings,  as  well  as  to  the  Art  Press  of  M.  Jaffe  in  Vienna  for  the  care- 
ful preparation  of  the  plates,  which  have  reproduced  the  characteristics 
of  the  original  drawings  in  the  truest  way. 

The  Author 
Vienna 
September,  igii 


CONTENTS 

PART   I.     ANATOMY   AND    HISTOLOGY   OF   THE   ADULT   EYEBALL 

PAGE 

A.  The  Eyeball  as  a  Whole  (Macroscopic  Anatomy) 3-16 

L     Form,  Size,  Orientation 3 

IL     Surface  of  the  Eyeball 7 

in.     General  View  of  the  Structure  of  the  Eyeball  (Division  into  Coats  and 

Zones) 10 

IV.     Asymmetry  of  the  Eyeball 16 

B.  Speclal  Anatomy  AND  Histology  OF  THE  E\-EBALL 17-1S6 

Chapter  I.     The  Sclera 17-26 

1.  The  Episcleral  Tissue 21 

2.  The  Sclera  Proper 22 

3.  The  Lamina  fusca  sclerae 26 

Chapter  II.     The  Cornea 26-40 

a)  The  Cornea  Proper 28 

1.  The  Epithelium  of  the  Cornea 28 

2.  Bowman's  Membrane 30 

3.  The  5«6.s/aH//a /'ro/)r/(z  of  the  Cornea 31 

4.  Descemet's  Membrane 36 

5.  The  Endothelium  of  the  Cornea        .      .' 37 

b)  The  Limbus  corneac 38 

1.  The  Epithelium  of  the  Limbus  corneae ^^ 

2.  The  Stroma  of  the  Limbus  corneae 35 

Chapter  III.    The  Structures  of  the  Scleral  Furrow 41-4S 

Schlemm's  Canal 41 

The  Meshwork  of  the  Iris  Angle 42 

Chapter  IV.    The  Perichorioidal  Space  and  the  Suprachorioidea  .  .       48-52 

Chapter  V.    The  Chorioidea 52-60 

1.  The  Vessel  Layer 54 

2.  The  Capillary  Layer 56 

3.  The  Glass  Membrane 5g 

Chapter  VI.    The  Pigment  Epithelium  of  the  Chorioidea 60-63 

Chapter  VII.    The  Retina 63-8S 

a)  ]\Iicroscopic  Anatomy  and  Histology  of  the  Retina 65 

1.  The  Layer  of  Rods  and  Cones 66 

2.  The  Membrana  limilans  externa 68 

3.  The  Outer  Nuclear  Layer 69 

4.  The  Outer  Plexiform  Layer 71 

5.  The  Inner  Nuclear  Layer        72 

6.  The  Inner  Ple.Kiform  Layer 73 

7.  The  Ganglion-Cell  Layer 74 


CONTENTS 

PACE 

8.  The  Nerve-Fiber  Layer 76 

Appendix.    The  Supporting  Fibers 78 

9.  Membrana  limilans  interna 79 

b)  Histologic  and  Functional  Divisions  of  the  Retina.     Its  Blood-Vessels 

and  Fovea  centralis 80 

c)  The  Extreme  Periphery  of  the  Retina 83 

Chapter  VIII.    The  Optic  Nerve 8S-107 

a)  The  Non-medullated  Portion  of  the  Ojjtic  Nerve 88 

1.  Microscopic  Anatomy  and  Histology 88 

2.  Varieties  of  the  Non-medullated  Section  of  the  Optic  Nerve       .      .  95 

3.  The  Significance  of  the  Ophthalmoscopic  Picture  in  Relation  to  the 
Anatomy  of  this  Region 97 

b)  The  Medullated  Section  of  the  Optic  Nerve 99 

1.  The  Sheaths  of  the  Optic  Nerve 100 

2.  The  Optic-Nerve  Trunk 103 

Chapter  IX.    The  Ciliary  Body 107-125 

a)  The  Uveal  Portion  of  the  Ciliary  Body 1 1 1 

1.  Suprachorioidea  and  Ciliary  Muscle iii 

2.  The  Vessel  Layer  of  the  Ciliary  Body 115 

3.  The  Elastic  Lamella 116 

4.  The  Interlamellar  Connective  Tissue 116 

5.  The  Cuticular  Lamella 117 

b)  The  Epithelial  Covering  of  the  Ciliary  Body 119 

6.  The  Pigment  Epithelium  of  the  Ciliary  Body 119 

7.  The  Ciliary  Epithelium 121 

8.  The  Membrana  limilans  interna  ciliaris 123 

Chapter  X.    The  Iris 125-148 

a)  The  Uveal  Portion  of  the  Iris 131 

1.  The  Endothelium  of  the  Iris 131 

2.  The  Anterior  Border  Layer 131 

3.  The  Vessel  Layer 133 

b)  The  Ectodermal  Layers  of  the  Lack  Surface  of  the  Iris 138 

4.  Outer  Leaf:  Dilatator  pupillac 138 

5.  Inner  Leaf:  Pigment  Epithelium  of  the  Iris 144 

c)  Variations  in  the  Appearance  of  the  Iris 145 

(Influence  of  the  Width  of  the  Pupil  and  Individual  Variations) 

Chapter  XL    The  Vitreous 148-15S 

Chapter  XII.    The  Zonula  ciliaris 155-162 

Chapter  XIII.    The  Lens 162-173 

1.  The  Lens  Capsule 164 

2.  The  Lens  Epithelium 167 

a)  The  Epithelium  of  the  Anterior  Lens  Surface 167 

;8)  The   Epithelial  Border  and  the  Lens  Vortex:   The  Formation  of 

New  Lens  Fibers 167 

3.  The  Lens  Substance 170 


CONTENTS  xi 

PAGE 

Chapter  XIV.    The  Chambers  of  the  Eyeball  and  the  Topography  of  This 

Region 173-1S0 

a)  The  Posterior  Chamber 1 73 

b)  The  Anterior  Chamber 176 

c)  Content  of  the  Chambers 177 

d)  Topography  of  the  Anterior  Segment 177 

Chapter  XV.    The  Vessels  and  Nerves  of  the  Eyeball 180-187 

a)  The  Blood-Vessels  of  the  Eyeball 180 

1.  The  Retinal  System 180 

2.  The  Ciliary  System i8r 

b)  Ljonph  Passages 185 

c)  Nerves  of  the  Eye  {N.  ciliares) 186 

PART  II.    THE   PHYSIOLOGIC   CHANGES   OF   THE   EYEB.VLL   DURING 
LIFE  (DEVELOPMENT  AND  SENESCENCE) 

Chapter  XVI.    The  Embryonal  and  Fetal  Development 191-203 

Chapter  XVII.    The  Eyeball  of  the  Newborn 203-206 

Chapter  XVIII.    The  Extrauterine  Development  and  Growth  of  the  Eyeball  207-211 

Chapter  XIX.     The  Appearances  of  Age  in  the  Eyeball 211-216 

Literature  Citations 217-226 

Explanation  of  Plate  Figures 227-232 


PART  I 
ANATOMY  AND  HISTOLOGY  OF  THE  ADULT  EYEBALL 


A.     THE  EYEBALL  AS  A  WHOLE  (MACROSCOPIC  ANATOMY) 
I.     Form,  Size,  Orientation 

(Text  Fig.  i) 

The  eyeball  {biilbus  ociili)  on  the  whole  has  the  form  of  a  sphere,  yet 
there  are  some  variations  from  a  pure  spherical  form. 

c 

Li 


Km/, 


Text  Fig.  i. — Right  eye,  schematic  horizontal  section.     Magnification  3. 

N  nasal,  T  temporal  side,  Co  conjunctiva  sclerae,  C  cornea,  Li  limbus,  Se  sulcus  sclerae  externus, 
5  sclera,  Rm  musculus  rectus  medialis,  Rl  musculus  rectus  lateralis,  /  iris,  Cc  corpus  ciliare.  Cor 
corona  ciliaris,  Or  orbiculus  ciliaris,  Mc  musculus  ciliaris,  Ch  chorioidea,  P  pigment  epithelium, 
R  retina,  F  fovea  centralis,  Os  ora  serrata,  Vk  anterior  chamber,  Hk  posterior  chamber,  L  lens,  Z  zonula 
ciliaris,  G  vitreous,  Lc  lamina  cribrosa,  0  nervus  opticus,  D  dural  sheath. 

To  begin  with,  one  notes  a  shallow,  circular  furrow  in  the  anterior 
segment  separating  a  smaller  transparent  area  of  about  1 2  mm  in  diameter 
(cornea,  C)  from  the  remaining  white  opaque  portion  (sclera,  S).  This 
furrow  {sulcus  sclerae  externus,  Se)  is  not  prominent  in  profile  view,  for 
it  is  filled  out  for  the  most  part  by  the  conjunctiva  sclerae  {Co) ;  one  recog- 
nizes it  better  if  one  allows  the  image  reflected  from  a  mirror  onto  the 
anterior  surface  of  the  cornea  to  move  toward  the  sclera.  A  narrowing 
of  the  image  then  occurs  in  the  horizontal  direction  (Tscherning,  227)  as 
the  neighborhood  of  the  margin  of  the  cornea  is  reached  and  an  elonga- 
tion or  a  division  into  two  images  is  seen  after  it  actually  passes  over  the 

3 


4  ANATOMY  AND  HISTOLOGY  OF  THE  HUMAN  EYEBALL 

margin  of  the  cornea.  The  latter  indicates  the  presence  of  a  concavity 
in  this  location.  This  concavity  is  somewhat  plainer  on  the  nasal  than 
on  the  temporal  side. 

The  cornea  is  more  sharply  curved  than  the  remainder  of  the  surface 
of  the  eyeball.  The  foremost  portions  of  the  sclera  are  very  weakly 
curved,  and  grade  off  very  abruptly,  almost  conically,  toward  the  equa- 
torial portion,  which  again  is  somewhat  more  sharply  curved. 

The  back  half  of  the  eyeball  has  a  more  uniform  curvature  than 
the  front  half,  yet  here,  too,  there  are  variations  from  the  pure  spherical 
form  and  I  cannot  accept  the  Merkel  schema  (151),  which  conceives 
of  the  form  of  the  back  half  as  that  of  a  sphere. 

In  the  first  place,  the  anatomic  equator,  i.e.,  the  sum-total  of  all  those 
points  maximally  distant  from  the  optic  axis,  does  not  go  through  an 
exact  frontal  plane,  but  lies  farther  forward  on  the  nasal  side,  farther 
backward  on  the  temporal  side.  Furthermore,  the  surface  of  the  eyeball 
lying  between  the  optic  nerve  and  the  nasal  part  of  the  equator  is  some- 
what flattened,  and  the  part  lying  temporal  to  the  optic  nerve  is  more 
strongly  curved  backward  and  outward. 

The  degree  of  this  asymmetry  varies  much.  In  many  cases  it  can 
only  be  made  out  by  the  accurate  comparison  of  the  profile  of  the  eyeball 
with  a  circle  of  the  same  diameter;  in  many  cases,  however,  it  is  so  strik- 
ing that  one  does  not  need  any  special  means  to  recognize  it. 

24  mm  may  be  looked  upon  as  the  normal  sagittal  or  long  diameter 
of  the  eyeball.  The  average  of  the  dimensions  given  by  various  authors 
is  as  follows: 

For  the  sagittal  diameter  24.26  mm 

For  the  transverse  diameter  23.7  mm 

For  the  vertical  diameter  23.57  mm 

The  majority  find  the  sagittal  diameter  to  be  the  greatest,  some, 
however,  e.g.,  Leopold  Weiss  (235),  the  transverse;  it  is  possible  that 
racial  peculiarities  are  responsible  for  this. 

The  limits  within  which  the  normal  eye  may  vary  have  been  deter- 
mined, especially  for  the  sagittal  diameter,  since  this  one  is  of  the  greatest 
importance  in  its  relation  to  refraction.  Schnabel  and  Herrenheiser  (190) 
found  an  axial  length  of  22.5  to  26mm  in  emmetropia.  However,  I 
doubt  whether  the  upper  limit  is  not  carried  too  far,  because,  according 
to  Elschnig  (52),  the  form  of  the  optic  nerve  entrance  characteristic  for 
the  slightly  myopic  eye  very  f  recjuently  makes  its  appearance  in  such  long 
emmetropic  eyes. 

Here  as  everywhere  else  the  normal  condition  goes  imperceptibly  over  into  the 
pathologic,  and  the  border  line  which  one  draws  between  the  two  is,  necessarily. 


FORM,  SIZE,  ORIENTATION  5 

somewhat  arbitrary  and  must  have  a  position  varying  with  the  individual  conception. 
In  my  opinion  the  finding  of  emmetropia  does  not  in  and  of  itself  guarantee  the 
normal  structure  of  the  eye.  The  normal  form  and  the  normal  internal  make-up  of 
the  eye  must  be  considered  as  well;  I  need  only  to  mention  here  the  operated 
myopic  eye,  which  can  be  emmetropic  under  favorable  circumstances  and  yet  possesses 
the  anatomic  evidence  of  high-grade  myopia  and  all  the  weakness  of  such  an  eye. 

The  eyeball  of  a  man  is  about  o .  5  mm  larger  in  all  dimensions  than 
that  of  a  woman.  Sappey  (186)  gives  the  following  figures  (in  the  same 
order  as  above) : 

For  the  eyeball  of  a  man  24 . 6  X  23 . 9  X  23 . 5  mm 

For  the  eyeball  of  a  woman  23.9X23.4X23  mm 

The  weight,  according  to  L.  Weiss  (235),  is  7  . 5  g,  the  volume  7  .  2  cm'. 

For  the  orientation  of  the  surface  of  the  eyeball  one  makes  use  of  the 
same  constructions  as  on  the  surface  of  the  earth. 

The  mid-point  of  the  cornea  determines  the  anterior  pole  of  the 
eyeball.  It  lies  diametrically  opposite  the  posterior  pole,  which  has  no 
other  anatomic  characteristic,  so  can  be  found  only  by  construction  or 
measurement.  The  line  of  union  between  the  two  poles  forms  the 
geometric  axis  of  the  eyeball. 

It  is  well  to  distinguish  this  from  the  optic  axis,  i.e.,  the  line  upon  which  the  focal 
points  of  the  refracting  surfaces  lie,  as  well  from  the  visual  line,  i.e.,  the  line  of  union 
between  the  fovea  centralis  and  the  nodal  point  of  the  optic  system.  In  the  strict 
mathematical  sense  an  optic  a.xis  exists  only  in  the  rarest  instances,  for  the  foci  of  the 
three  most  important  refracting  surfaces  (anterior  corneal,  anterior  and  posterior  lental) 
do  not  lie  upon  one  and  the  same  straight  line  at  all  as  a  rule  (Zeeman,  244) .  The  \dsual 
line  bears  away  strongly  from  the  geometric  axis  in  any  case,  for  the  fovea  lies  temporal 
to  and  below  the  posterior  pole. 

If  one  measures  the  geometric  axis  from  the  anterior  surface  of  the 
cornea  to  the  posterior  surface  of  the  sclera,  it  may  also  well  be  called 
the  outer  axis;  if,  however,  one  measures  only  to  the  light-perceiving 
layer,  i.e.,  to  the  outer  surface  of  the  retina,  one  calls  this  dimension 
the  inner  axis.  It  is  this  dimension  which  comes  into  consideration  in 
the  refraction  of  the  eye. 

Those  circles  which  can  be  drawn  through  both  poles  are  called 
meridians;  the  equator  is  that  circle  which  is  equidistant  from  the  two 
poles. 

This  is  the  geometric  equator  and  according  to  the  above  statements  does  not 
coincide  with  the  anatomic  equator. 

A  section  through  the  vertical  meridian  divides  the  eyeball  into  a 
nasal  or  medial,  and  a  temporal  or  lateral  half.  The  expressions  "inner" 
and  ''outer"  should  never  be  used  in  the  sense  of  medial  and  lateral,  but 
only  as  follows:    inner  is  that  which  lies  nearer  the  mid-point  of  the 


6  ANATOMY  AND  HISTOLOGY  OF  THE  HUMAN  EYEBALL 

eye,  outer  that  which  lies  nearer  the  surface.  The  expressions  "forward" 
and  "backward"  do  not  refer  to  the  sagittal  direction  alone  but  to  the 
meridian  as  well :  forward  is  that  which  lies  nearer  the  anterior  pole. 

For  example,  the  edge  of  the  ciUary  body  bordering  upon  the  trabeculum  of  the 
iris  angle  is  called  the  anterior,  that  bordering  upon  the  chorioidea,  the  posterior  border, 
despite  the  fact  that  the  difference  in  position  in  a  sagittal  direction  is  much  less  than 
in  a  frontal  one.  But  if  one  were  to  refer  to  parts  according  to  their  actual  position, 
the  description  would  become  extremely  inconstant  and  confused. 

Since  we  mainly  make  use  of  sections  to  illustrate  anatomic  relation- 
ships, it  may  be  well  to  say  something  here  concerning  the  direction  of 
sections  and  what  they  are  called. 

The  most  important  section  direction  is  the  meridional,  i.e.,  a  section  in 
the  plane  of  a  meridian.  Of  the  various  meridians,  the  horizontal  one  comes 
mainly  into  consideration  for  anatomic  purposes.  It  contains  the  most 
details,  and  horizontal  sections,  as  they  are  called  for  short,  are,  therefore, 
the  most  instructive.  Other  meridians  are  only  chosen  for  special  purposes. 
Frontal  or  equatorial  sections  are  sections  parallel  to  the  equatorial  plane. 

In  most  cases,  however,  a  direction  of  sectioning  must  be  chosen 
which  does  not  coincide  with  any  of  these.  In  this  case  when  the  section 
is  made  at  right  angles  to  the  meridian  and  at  right  angles  to  the  surface 
of  the  bulb,  I  call  it  a  transverse  section,  for  it  stands  in  the  same  rela- 
tion to  a  meridional  section  of  the  area  affected  as  does  the  cross-section 
to  a  longitudinal  section. 

When,  on  the  other  hand,  the  section  falls  parallel  or  tangential  to 
the  surface,  it  is  called  a  surface  section.  Since,  however,  most  of  the 
surfaces  of  the  bulb  are  curved,  only  tangential  sections  can  be  made,  and 
in  such  sections  the  tissue  is  actually  cut  along  the  surface  over  only  one 
small  area ;  farther  away  from  this  place  the  sections  become  increasingly 
oblique  (the  same  holds  true  for  transverse  sections). 

For  purely  histologic  purposes  such  surface  sections  are  usually 
very  useful,  for  even  the  area  which  is  actually  cut  along  the  surface 
contains  a  very  considerable  number  of  tissue  elements.  On  the  other 
hand,  surface  sections  are  inadequate  for  anatomic  purposes,  in  which 
the  general  view  is  more  sought  for,  and  must  be  replaced  by  surface 
preparations,  i.e.,  by  thin  sections,  which  are  only  obtained  by  anatomic 
preparations. 

Today  there  is  a  tendency  to  study  sections  only,  because  the  modern  staining 
methods  lend  themselves  better  to  sections,  and,  indeed,  sections  give  very  beautiful 
and  instructive  pictures.  Above  all,  the  modern  section  methods  contribute  an  incom- 
parable amount  to  the  topography.  The  older  methods  of  anatomic  preparation 
and  the  teased  preparations  do  not,  however,  by  any  means  deserve  the  disregard  which 
they  today  receive.     In  the  first  place  they  show  us  the  tissue  elements  in  a  much  more 


SURFACE  OF  THE  EYEBALL  7 

natural  state,  and  warrant  histologic  conclusions  far  better  than  do  cut  preparations. 
Generally,  one  can  very  well  combine  modern  staining  with  anatomic  preparations. 

The  making  of  surface  and  teased  preparations  demands  of  course  much  more  care 
and  skill  than  do  cut  preparations,  and  one  must  also  reckon  with  a  large  percentage 
of  failure.  But  one  successful  preparation  of  this  kind  may  give  data  which  one  seeks 
in  vain  in  hundreds  of  cut  sections.  So  a  thorough  knowledge  of  anatomic  and  histo- 
logic relationships  is  only  to  be  had  by  the  use  of  both  the  older  and  newer  methods. 

The  physical  conception  of  the  structures  and  tissue  elements  is,  above  all,  the 
fruit  of  this  combination  of  methods.  It  should  be  the  first  and  most  important  object 
of  histo-anatomic  study.  I,  at  least,  consider  it  one  of  the  main  objects  of  teaching  to 
convey  a  correct  physical  conception. 

II.     Surface  of  the  Eyeball 
(Text  Fig.  2) 
In  the  examination  of  the  outer  surface  of  the  eyeball  the  cornea, 
in  front,  first  presents  itself  (Text  Fig.  2  a,  C).     Its  relation  to  the  sulcus 
sclerae  externus  has  already  been  spoken  of.     The  cornea  is  slightly  ellip- 
tical, for  the  horizontal  diameter  is  greater  than  the  vertical.     The 


N       N 


Text  Fig.  2. — Topography  of  the  surface  of  the  eyeball  (right  eye).    Magnification  1.5. 

a  view  in  front,  b  from  behind,  c  from  above.  N  nasal,  T  temporal  side,  C  cornea,  0  optic  nerve, 
Rm  line  of  insertion  of  the  musculus  rectus  medialis,  Rl  line  of  insertion  of  the  musculus  rectus  lateralis, 
Rs  line  of  insertion  of  the  musculus  rectus  superior,  Ri  line  of  insertion  of  the  musculus  rectus  inferior, 
Os  line  of  insertion  of  the  musculus  obliquus  superior,  Oi  line  of  insertion  of  the  musculus  obliquus  infe- 
rior. Am  arteria  ciliaris  posterior  longa  medialis,  Al  arteria  ciliaris  posterior  Icnga  lateralis,  V,  W  T'j  V^ 
venae  vorticosae.  The  dotted  lines  in  a  and  b  indicate  the  two  main  meridians,  in  c  the  vertical  meridian 
and  the  geometric  equator. 

conjunctiva  sclerae  continues  over  into  the  outer  layers  of  the  cornea  and 
is  also  more  firmly  adherent  to  the  sclera  over  the  floor  of  the  sulcus 
sclerae;  farther  back,  however,  it  is  united  to  the  sclera  by  very  loose 
connective  tissue  only.  One  is  therefore  compelled  to  leave  a  strip 
(stump)  of  conjunctiva,  i  mm  or  so  in  length,  on  the  bulb  at  the  time  of 
enucleation.  This  stump  contracts  and  in  the  anatomic  preparation 
usually  appears  somewhat  prominent  or  bulging.  This  tumifaction  is 
therefore  an  artefact;  when  all  the  parts  are  /;/  situ  and  uninjured,  there 


8  ANATOMY  AND  HISTOLOGY  OF  THE  HUMAN  EYEBALL 

is  no  sudden  change  of  level  visible  in  the  transition  from  the  conjunctiva 
onto  the  cornea. 

Farther  away  from  the  margin  of  the  cornea  but  still  in  the  anterior 
half  of  the  bulb,  one  comes  upon  the  lines  of  insertion  of  the  recti  muscles. 

The  following  measurements  are  taken  from  Fuchs  (65).  They  are 
the  averages  found  in  31  emmetropic  eyes. 

Text  Fig.  2,  given  herewith,  does  not  agree  exactly  with  these  measurements;  the 
figure  presents  a  concrete  case.  Furthermore,  a  perspective  foreshortening  comes  out 
in  the  marginal  portions;  therefore  the  insertion  lines  appear  concave  while  in  reality, 
i.e.,  when  viewed  at  right  angles  to  the  surface,  they  are  straight  or  weakly  convex. 

The  insertion  lines  of  the  m.  rectus  mcdialis  (Rm)  and  w.  rectus  late- 
ralis (Rl)  are  vertical,  straight,  and  usually  symmetrical  to  the  hori- 
zontal meridian  (although  that  of  the  medialis  lies  somewhat  lower, 
the  lateralis  somewhat  higher). 

The  insertion  lines  of  the  m.  rectus  superior  (Rs)  and  m.  rectus 
inferior  (Ri)  are  weakly  conve.x  forward  and  lie  in  an  oblique  plane  so 
that  the  nasal  end  of  the  insertion  lies  nearer  the  cornea  than  the  tem- 
poral end.  Both  lines  of  insertion  are,  moreover,  shifted  somewhat 
temporal. 

The  distance  from  the  margin  of  the  cornea  and  the  breadth  of  the 
tendon  (or  length  of  the  line  of  insertion)  is  shown  in  the  following  table : 

Distance  from  the  Cornea      Width  of  Tendon 


M.  rectus  mcdialis. 
M.  rectus  lateralis.. 
M.  rectus  superior. 
M.  rectus  inferior.  . 


5.5  mm 
6.9 


6.5 


10.3  mm 

9.2 
10.6 

9.8 


The  insertion  lines  of  the  m.  obliqui  lie  in  the  back  half  of  the  bulb — 
that  of  the  m.  obliquus  superior  above,  that  of  the  m.  obliquus  inferior 
temporal  (Te.xt  Fig.  2,  b,  c). 

The  insertion  line  of  the  m.  obliquus  superior  {Os)  forms  a  10.7  mm 
long  bow  with  convexity  backward.  Its  anterior  end  lies  in  about  the 
same  meridian  as  the  temporal  end  of  the  insertion  of  the  rectus  superior. 
The  greater  part  (sometimes  the  whole)  of  the  insertion  line  lies  temporal 
to  the  vertical  meridian ;  the  angle  which  it  forms  with  the  vertical  merid- 
ian measures  on  the  average  45°,  but  this  is  subject  to  wide  variation. 
According  to  Fuchs,  one  can  distinguish  two  types  of  insertion  lines,  the 
one  having  a  more  equatorial,  the  other  a  more  meridional  direction. 

The  m.  obliquus  inferior  [Oi)  has  the  shortest  tendon  of  all  the  eye 
muscles  (often  practically  none  at  all).     One,  therefore,  often  sees  cross- 


SURFACE  OF  THE  EYEBALL  9 

sections  of  muscle-fibers  clinging  to  the  outer  surface  of  the  sclera  on 
the  temporal  side  in  horizontal  sections  through  the  posterior  half  of  the 
eye;  they  belong  to  the  m.  obliqmis  iiiferior,  and  give  one  data  as  to  which 
is  the  temporal  side.  The  insertion  line  is  9 .4  mm  long  and  forms  a  bow 
with  its  convexity  upward,  but  often  shows  gross  irregularities,  such  as 
angular  serrations  or  dehiscences.  It  lies  for  the  most  part  below  the 
horizontal  meridian  and  makes  an  angle  of  some  19°  with  it.  The  pos- 
terior (nasal)  end  of  the  insertion  comes  to  within  5  mm  of  the  sheath 
of  the  optic  nerve,  the  anterior  (temporal)  end  lies  in  about  the  same 
meridian  as  the  lower  end  of  the  insertion  of  the  rectus  lateralis. 

The  optic  nerve  (0),  with  its  sheaths,  forms  a  triangular  rounded 
cord  some  5  mm  in  diameter.  It  is  inserted  to  the  nasal  side,  so  that 
the  center  of  its  insertion  surface  lies  some  3  mm  mesial  to  and  i  mm 
below  the  posterior  pole. 

On  both  sides  of  the  optic  nerve  (nasal  and  temporal  to  it)  the  arteriae 
ciliares  posteriores  longae  are  visible  as  bluish  stripes.  They  hold  pretty 
closely  to  the  line  of  the  horizontal  meridian  and  their  line  of  union,  there- 
fore, goes  slightly  above  the  axis  of  the  optic  nerve.  The  point  of 
entrance  of  the  arteria  ciliaris  posterior  longa  medialis  {Am)  lies  some 
3 . 6  mm  from  the  optic  nerve,  somewhat  nearer  the  optic  nerve  than  does 
that  of  the  a.  c.  p.  I.  lateralis  {Al),  3  .9  mm  away. 

The  vortex  veins  {venae  vorticosae)  are  usually  four  in  number  and 
lie  grouped  in  two  pairs  (an  upper  and  lower) .  The  points  of  exit  of  the 
upper  pair  lie  on  the  respective  sides  of  the  vertical  meridian,  displaced 
somewhat  nasally,  and  7  mm  (the  superior  nasal  vein,  F,)  to  8  mm  (the 
superior  temporal  vein,  FJ  behind  the  equator.  The  latter  lies  very 
close  to  the  insertion  of  the  m.  obliqmis  superior.  The  lower  pair  {V^,  FJ 
show  a  similar  relationship  to  the  vertical  meridian,  but  lie,  however, 
somewhat  farther  forward  (5 . 5  to  6  mm  behind  the  equator). 

The  decision  as  to  whether  an  eyeball  is  the  right  or  left  one  is  made  by  the  aid  of 
the  back  segment,  because  in  the  anterior  segment  one  does  not  notice  difference 
enough  between  things  in  the  upper  and  lower  portions. 

First  one  determines  the  horizontal  meridian :  anteriorly,  by  the  long  axis  of  the 
cornea,  posteriorly,  by  the  position  of  the  optic  nerve  and  the  long  posterior  ciliary 
arteries.  Then  one  searches  for  the  lines  of  insertion  of  the  obliques.  The  one 
reaching  closer  to  the  optic  nerve,  lying  snugly  on  a  long  posterior  ciliary  artery,  pro- 
vided with  a  short  tendon  (or  none  at  all),  is  the  obliquus  inferior  and  belongs  on  the 
temporal  side.  The  other,  lying  farther  from  the  optic  nerve  and  the  ciliary  artery, 
pro\'ided  with  a  longer  tendon  of  insertion,  belongs  above.  The  bulb  is,  therefore,  to 
be  so  oriented  that  the  piece  lying  between  the  two  insertion  lines  corresponds  to  the 
upper  temporal  quadrant ;  thereby  the  correct  position,  and,  moreover,  the  side  to  which 
the  eyeball  belongs,  is  found. 


10  ANATOMY  AND  HISTOLOGY  OF  THE  HUMAN  EYEBALL 

in.     General  View  of  the  Structure  of  the  Eyeball 

{Division  into  Coats  and  Zones) 

For  the  study  of  the  grosser  anatomy  it  is  recommended  to  divide 
the  eyeball,  fresh  or  preserved  in  a  very  weak  solution  of  formalin,  into 
two. 

The  division  may  be  made  in  a  meridional  or  equatorial  direction. 
Since  the  lens  is  easily  dislocated  in  meridional  cutting,  it  is  well  to 
freeze  the  eye  before  cutting  it.  The  equatorial  section  can  be  made 
without  any  special  preparation;  one  carries  the  section  as  far  as  it  will 
go  with  a  thin  sharp  knife  and  completes  it  with  scissors. 

It  is  at  once  seen  that  the  eyeball  consists  of  a  firm  wall  and  a  softer, 
transparent  contents. 

We  will  lirst  consider  the  inner  surface  of  the  wall  of  the  eyeball  in 
an  equatorially  halved  eye. 

The  posterior  segment  of  a  wholly  fresh  eye  presents  about  the  same 
picture  as  with  the  ophthalmoscope  except  that  the  eyeground  does  not 
appear  red  but  brown.  One  recognizes  the  entrance  of  the  optic  nerve 
as  a  circular  or  slightly  oval  disc  of  white  color  (PL  VII,  i).  The 
retinal  vessels  are  empty  or  the  veins  filled  with  broken  columns  of  blood; 
therefore,  one  sees  only  the  larger  branches.  The  region  of  the  fovea 
centralis  comes  out  temporal  to  the  optic  nerve  entrance  by  its  darker 
color.     Toward  the  equator  the  markings  of  the  vortices  are  seen  (PI. 

ni,  5)- 

The  retina  is  wholly  transparent  in  a  fresh  state,  but  a  cadaverous 
clouding  soon  occurs.  This  usually  appears  first  in  the  region  of  the  fovea 
and  at  the  same  time  this  region  usually  becomes  detached  and  folded. 
Simultaneously  the  yellow  fleck  (macula  liitea)  comes  forth  in  this  region. 
In  older  cadaverous  eyes  the  retina  is  completely  opaque  and  usually 
detached,  and  the  eyeground,  therefore,  shows  a  gray  instead  of  a  brown 
color. 

In  the  anterior  segment  (PL  II,  i)  one  notes  first  in  the  cut  surface 
the  jagged  border  of  the  retina  {ora  serrala,  Os) ;  it  is  much  better  seen 
in  the  cadaverous  than  in  the  fresh  eye,  because  of  the  clouding  of  the 
retina.  The  zone  lying  in  front  of  the  ora  serrata  is  considerably  darker 
than  that  behind  it,  has  no  irregularities  visible  to  the  naked  eye,  and  is 
known  as  the  orbiculiis  ciliaris  {Or).  This  zone  is  4  mm  wide  on  the 
average.  Toward  the  lens  it  is  succeeded  by  a  crown  of  whitish,  plainly 
prominent,  radiating  stripes:  these  stripes  are  the  ciliary  processes  and 
the  whole  zone  (1.5  to  2  mm  wide)  is  called  the  corona  ciliaris  (Cor). 
Upon  this  follows  the  uniformly  dark-brown  back  surface  of  the  iris. 
The  lens  (L)  must  be  first  removed  if  one  wishes  to  bring  this  into  plain 


GENERAL  VIEW  OF  THE  STRUCTURE  OF  THE  EYEB.\LL  ii 

view.  The  lens  is  a  circular,  disc-form  structure,  some  9  mm  in  diameter. 
Between  it  and  the  corona  ciliaris  is  a  narrow  interval  (scarcely  0.5  mm 
broad),  the  circumlental  space  (CI),  bridged  over  by  the  fiber-bundles 
of  the  zonula  ciliaris. 

In  this  way  one  can  make  out  three  main  zones  in  the  wall  of  the 
eyeball. 

The  posterior  main  zone  extends  from  the  optic-nerve  entrance  to 
the  ora  scrrala  retinae;  the  middle  zone  comprises  the  orbiculus  and 
corona  ciliaris,  the  anterior  zone  includes  the  iris  on  the  inner,  the  cornea 
on  the  outer,  surface  of  the  eyeball. 

The  wall  of  the  eye  also  shows  a  number  of  main  layers  (tunics); 
these  are  especially  easy  to  make  out  in  the  posterior  main  zone  and 
can  be  made  into  anatomic  preparations. 

Outside,  there  is  a  thick,  firm  coat,  whose  posterior  white  segment 
we  know  by  the  name  sclera,  and  whose  anterior  transparent  portion  we 
have  already  come  to  know  as  the  cornea;  for  the  two  together,  i.e., 
the  whole  coat,  the  old  name  tunica  fibrosa  (Text  Fig.  1,  S,  C)  is  best 
adapted.  Then  follows  a  more  delicate,  brown,  vessel-rich,  coat,  the 
tunica  vasculosa  s.  uvea  (Text  Fig.  i,  C/i,  Cc,  I).  On  the  inner  surface 
of  this  coat  there  lies  an  extremely  thin  coat  consisting  microscopically 
of  a  single-celled  layer,  which,  likewise,  is  strongly  pigmented,  the  stratum 
pignienti  (Text  Fig.  i,  P).  Finally,  inside  this  there  lies  the  retina 
(Text  Fig.  I,  R).  This  is  a  part  of  the  tunica  interna;  it  is  transparent 
during  life  except  for  its  blood-vessels,  and  after  death  is  lightly  clouded 
and  mostly  detached. 

The  walls  of  the  eyeball  are,  therefore,  made  up  of  four  principal 
layers;  from  without  inward  there  are:  (i)  tunica  fibrosa,  (2)  tunica 
vasculosa  s.  uvea,  (3)  stratum  pigmenti,  (4)  tunica  interna. 

The  above-described  main  zones  form  from  unequal  development 
(differentiation)  of  the  tunica  interna  and  the  stratum  pigmenti.  In  the 
tunica  vasculosa  the  zone  borders  are  not  sharp  and  in  the  tunica  fibrosa 
only  partly  expressed. 

For  the  purpose  of  the  more  detailed  description  we  will  take  up  the 
individual  parts  of  the  wall  of  the  eyeball  from  without  inward  and  from 
behind  forward. 

a)    The  Wall  of  the  Eyeball 
I.     The  tunica  fibrosa 

This  tough  fibrous  capsule  of  the  eyeball  is  closed  on  all  sides;  it 
lends  form  and  size  to  the  eyeball,  its  firmness  protects  the  delicate  inner 
portions  from  insult.  At  the  optic-nerve  entrance  its  outer  layers  go  over 
into  the  optic-nerve  sheaths  (Text  Fig.  i,  D).     Its  inner  layers  show 


12  ANATOMY  AND  HISTOLOGY  OF  THE  HUMAN  EYEBALL 

a  round  hole  {foramen  opticum  sclcrac).  This  is  incomplelely  closed  by  a 
sieve-like  perforated  plate  {lamina  cribrosa,  Text  Fig.  i,  Lc).  Other- 
wise the  continuity  of  the  tunica  fibrosa  is  broken  only  by  line  canals 
(emissaria) ,  which  contain  vessels  and  nerves  going  to  the  tunica  vasculosa. 
One  can  make  out  only  two  zones  in  the  tunica  fibrosa.  The  one 
corresponds  to  the  middle  and  posterior  zones  of  the  bulb;  here  the 
tunic  is  white  and  opaque  (sclera,  Text  Fig.  i,  5).  The  other  zone 
corresponds  to  the  anterior  zone  of  the  bulb;  here  the  coat  is  trans- 
parent and  shining  (cornea,  Text  Fig.  i,  C).  This  part  is,  moreover,  an 
integral  part  of  the  optical  system. 

2.     The  iiinicii  vasculosa  s.  uvea 

This  is  the  main  organ  of  nutrition  of  the  eyeball  and  the  bearer 
of  the  intraocular  musculature.  It  consists  mainly  of  blood-vessels; 
the  connective  tissue  system  is  poorly  developed  and  colored  brown  by 
the  richly  branched  pigment  cells  (chromatophores). 

The  tunica  vasculosa  lies  snugly  against  the  sclera,  yet  it  is  only  grown 
to  it  at  two  places.  These  places  have  the  forms  of  rings  and  may  be 
designated  from  their  position  as  the  posterior  and  anterior  insertion  rings. 

The  posterior  insertion  ring  lies  about  the  optic  nerve;  the  two  tunics 
are  here  directly  bound  to  one  another.  The  anterior  insertion  ring 
lies  on  the  border  of  the  middle  and  anterior  zones  of  the  bulb  (the 
corneoscleral  border).  The  union  of  the  two  coats  is  partly  direct,  partly 
by  means  of  a  peculiar  meshwork. 

Between  these  two  rings  there  extends  a  capillary  space  (the  peri- 
chorioidal  space),  crossed  only  by  a  few  blood-vessels  and  nerves,  and 
bridged  by  extremely  delicate  tissue  lamellae  {supracliorioidea).  One 
can,  therefore,  demonstrate  the  entire  expanse  of  the  tunica  vasculosa 
with  relative  ease. 

The  posterior  insertion  ring  is  the  most  difficult  to  detach ;  this  region  must  eventu- 
ally be  cut  away.  The  anterior  insertion  ring  can  be  very  easily  detached  by  means  of 
blunt  instruments;  one  then  only  needs  to  cut  the  efferent  and  afferent  blood-vessels  and 
the  ciliary  nerves  and  the  tunica  vasculosa  is  entirely  separated  from  the  tunica  fibrosa. 

One  also  recognizes  only  two  zones  on  the  outer  surface  of  the  tunica 
vasculosa,  for  here  the  border  between  the  posterior  and  the  middle  zone 
is  not  visible ;  both  appear  uniformly  brown,  and,  when  the  preparation 
lies  in  water,  provided  with  fine  floating  brown  shreds  (suprachorioidal 
lamellae).  Some  meridional  white  bands  stand  out  against  the  brown 
background :  these  are  the  ciliary  nerves  coursing  forward  along  the  outer 
surface  of  the  tunica  vasculosa. 

A  broader  strand  is  present  in  the  horizontal  meridian,  nasal  as  well 
as  temporal;  more  accurately  studied,  this  is  seen  to  be  made  up  of  three 


GENERAL  VIEW  OF  THE  STRUCTURE  OF  THE  EYEBALL     13 

Stripes:  the  central  one  is  an  artcria  ciliaris  postior  louga;  the  two  by 
its  side  are  nerves. 

Forward,  all  these  meridional  bands  sink  into  the  ciliary  muscle; 
the  muscle  itself  is  inserted  directly  into  the  anterior  insertion  ring  as  a 
whitish  girdle  about  3  mm  broad.  For  the  description  of  the  anterior 
zone  of  the  tunica  vasculosa,  see  the  iris  (chap.  x). 

The  tunica  vasculosa  shows  two  round  holes:  one  behind  for  the 
entrance  of  the  optic  nerve  (foramen  opticum  cliorioideae),  corresponding 
in  its  position  exactly  to  foramen  opticum  sclerae,  and  one  anterior  for 
the  entrance  of  light  (pupil). 

3.  The  stratum  pigmcnti  and 

4.  The  tunica  interna 

These  are  present  in  exactly  the  same  expanse  as  the  tunica  vasculosa 
and,  in  general,  intimately  united  to  it.  In  their  differentiation  these 
three  coats  are  so  dependent  upon  one  another  that  it  is  best  to  continue 
their  description  as  zones  rather  than  as  coats. 

a)    POSTERIOR    ZONE 

The  tunica  vasculosa  here  forms  the  cJwrioidea  (Text  Fig.  i,  Ch), 
This  serves  mainly  as  the  nutritional  source  for  the  outer  layers  of  the 
retina  and  for  this  reason  possesses  a  distribution  of  blood-vessels  of 
its  own. 

The  stratum  pigmcnti  is  made  up  of  a  smooth,  regular,  easily  detach- 
able cell-layer  (pigment  epithelium  of  the  chorioidea,  Text  Fig.  i,  P), 

The  stratum  pigmenti  belongs  developmentally  to  the  tunica  interna  (cf.  chap,  -xvi), 
but  anatomically  it  is  classified  with  the  tunica  vasculosa;  especially  in  cadaver-eyes  does 
the  pigment  epithelium  of  the  chorioidea  appear  as  a  covering  of  the  latter,  whereas 
only  the  corresponding  portion  of  the  tunica  interna  (the  retina)  is  detached.  But  the 
addition  of  "  the  chorioidea ''  to  this  term  should  never  be  used  to  mean  more  than  the 
zone  in  this  case. 

The  tunica  interna  is  highly  differentiated  in  this  zone  and  developed 
into  a  regular  stratified  coat  of  nerve  tissue  (retina.  Text  Fig.  i,  R), 
which  functionates  as  the  organ  for  the  reception  of  visual  impressions. 

The  term  retina  is  used  by  authors  in  many  different  senses.  We  use  it  in  the 
narrower  sense,  i.e.,  to  designate  that  membrane  which  receives  the  visual  impressions. 
By  many  authors  the  retina  is  synonjTnous,  however,  with  the  tissues  arising  out  of  the 
optic  vesicle,  and  the  term,  therefore,  is  used  in  the  wider  sense  of  the  word.  In  this 
sense  the  stratum  pigmenti  is  also  included  and  the  posterior  zone  is  then  designated  as 
the  pars  optica  retinae.  I  will  not  dispute  the  justification  for  such  a  conception  of  the 
term  retina,  but  I  think  that  it  is  more  to  the  purpose  to  use  the  word  retina  in  the 
narrower  sense  with  the  beginner,  for  it  is  almost  always  used  in  this  narrower  sense 
in  ophthalmology  and  pathology. 


14  ANATOMY  AND  HISTOLOGY  OF  THIC  HUMAN  EYEBALL 

/3)    MIDDLE   ZONE 

Since  one  cannot  separate  the  stratum  pigmcnli  and  liiiiica  iiitcr)ia 
from  the  tunica  vasculosa  macroscopically  in  this  zone,  one  designates 
the  totahty  of  all  three  coats,  in  so  far  as  they  belong  to  the  middle  zone, 
as  the  ciliary  body  {corpus  ciliare,  Text  Fig.  i,  Cc). 

According  to  the  division  of  its  inner  surface,  the  ciliary  body  is 
divided  into  two  adjacent  zones,  about  which  we  have  learned  above 
(p.  lo);  the  posterior  one  is  the  orbicidus  ciliaris,  or  fiat  portion  {pars 
plana,  Or),  the  other  the  corona  ciliaris,  or  folded  portion  {pars  plicata, 
Cor). 

The  ciliary  body  is  the  organ  for  the  nourishment  of  the  vitreous  and 
lens,  secretes  the  aqueous,  and  performs  the  act  of  accommodation. 

The  main  mass  of  the  ciliary  body  belongs  to  the  tunica  vasculosa 
and  is  designated  as  the  pars  uvealis  corporis  ciliaris.  Like  the  chorioidea, 
it  is  rich  in  blood-vessels,  yet  these  show  another  distribution.  Above 
all,  however,  it  is  characterized  by  the  ciliary  muscle  {Mc). 

The  stratum  pigmenti  adheres  firmly  to  the  inner  surface  of  the  pars 
uvealis  c.f.,  and,  as  in  the  chorioidea,  is  a  single  layer  of  pigmented  cells 
although  of  slightly  different  characteristics  than  there.  It  should  be 
designated  as  the  pigment  epithelium  of  the  ciliary  body. 

The  tunica  interna  is  reduced  to  a  simple  layer  of  unpigmented  cells 
(non-pigmented  ciliary  epithelium,  or  ciliary  epithelium  for  short). 

Those  who  use  the  expression  retina  in  the  wider  sense  call  the  pigment  epithelium, 
together  with  the  non-pigmented  ciliary  epithelium,  the  pars  ciliaris  retinae. 

y)    ANTERIOR   ZONE 

The  totality  of  all  three  coats  {tunica  vasculosa,  stratum  pigmenti, 
and  tunica  interna),  so  far  as  they  belong  to  the  anterior  zone,  bear  the 
name  iris  (Text  Fig.  i,  /).  Its  most  important  function  is  that  of  a 
diaphragm,  and  for  this  reason  it  is  suspended  entirely  free  in  the  interior 
of  the  eye,  i.e.,  lies  separated  from  the  tunica  fibrosa  by  a  wide  space, 
the  anterior  chamber  (1'^),  and  is  provided  with  a  round  opening  of 
changeable  width  (pupil).  The  tu)iica  vasculosa  ends  at  this  opening, 
and  the  stratum  pigmenti  goes  over  into  the  tujiica  /«/(T»a  (transition  point 
or  border  of  the  optic  cup;  cf.  chap.  xvi). 

The  tunica  vasculosa  portion  of  the  iris  is  called  the  pars  uvealis 
iridis;  the  stratum  pigmenti  is  differentiated  into  an  epithelial  muscle 
{dilatator  pupillae) ;  the  tunica  interna,  as  in  the  ciliary  body,  has  a  pure 
epithelial  character;  it  takes  on  here,  however,  the  function  of  making  the 
interior  of  the  eye  dark  and  appears,  therefore,  as  the  pigment  epithelium 
of  the  iris. 


GENERAL  VIEW  OF  THE  STRUCTURE  OF  THE  EYEBALL     15 

When  the  term  retina  is  used  in  the  wide  sense,  the  dilatator  pupillae  and  pigment 
epithelium  of  the  iris  form  the  pars  relinalis  iridis,  s.  pars  iridica  retinae. 


Anterior  Zone 

JIiDDLE  Zone 

Posterior  Zone 

1.  Tunica  fibrosa 

Cornea 

Sclera 

Anterior  chamber 

Perichorioidal  space  (suprachorioidea) 

2.  Tunica  vasculosa 

'C 

Pars  uvealis 

iridis 

—  Sphincter  pup. 

3 

6 

Pars  uvealis 
corporis  ciliaris 

Chorioidea 

3.  Stratum  pigment! 

Dilatator 

pupillae 

(+Sphincter 

pup.) 

0^ 

Pigment 

Epithelium  of  the 

Ciliary  Body 

Unpigmented 

Ciliary 

Epithelium 

Pigment 

Epithelium  of  the 

Chorioidea 

ni 

D. 

4.  Tunica  interna 

Pigment 

Epithelium  of 

the  Iris 

Retina 
(sensu  strictiori) 

In  the  above  chart  I  have  sought  to  give  a  survey  of  the  division  of 
the  eyeball  into  coats  and  zones  with  the  idea  of  making  the  connection 
of  the  various  parts,  as  well  as  the  somewhat  confused  nomenclature,  more 
easily  understood. 

Only  the  following  need  be  said  in  advance  about  the  developmental 
history  of  the  eyeball :  coats  3  and  4  come  from  the  primary  optic  vesicle 
and  the  tissues  belonging  to  these  coats  are,  therefore,  of  ectodermal 
origin;  layers  i  and  2  arise  from  mesoderm;  only  the  blood-vessels 
(mesoderm)  of  the  retina  and  the  epithelium  of  the  cornea  (ectoderm) 
constitute  exceptions  to  the  above  rule;  the  same  is  true  of  the  deposition 
of  ectodermal  elements  {sphincter  pupillae)  in  the  iris  mesoderm. 

b)  The  Contents  of  the  Eyeball 

The  vitreous  [corpus  vitrcum,  Text  Fig.  i,  G)  forms  the  main  mass  of 
and  completely  fills  out  the  interior  space  encompassed  by  the  posterior 
zone,  and  that  surrounded  by  the  middle  zone,  partially.  Behind  and 
to  the  sides  it  lies  against  the  inner  surface  of  the  retina ;  in  front  it  pre- 
sents a  concavity  {fossa  patellaris)  in  which  the  lens  {L)  lies.  This  is 
mainly  held  in  its  place  behind  the  pupil  by  the  zonula  ciliaris  (Z),  a  fiber 
system  given  off  from  the  inner  surface  of  the  ciliary  body. 

The  space  yet  remaining  is  filled  out  by  aqueous,  and  is  divided  by  the 
iris  into  a  posterior  {Hk)  and  an  anterior  chamber  (I'^). 


i6  ANATOMY  AND  HISTOLOGY  OF  THE  HUMAN  EYEBALL 

IV.     Asymmetry  of  the  Eyeball 

When  looked  at  from  the  outside  the  asj'mmetry  of  the  eyeball  is  at 
once  apparent  in  the  unequal  remoteness  of  the  lines  of  insertion  of  the 
eye  muscles,  in  the  oblicjue  position  of  the  anatomatic  equator,  and 
especially  in  the  entrance  of  the  optic  nerve  to  one  side. 

The  interior  of  the  eye  also  shows  this  asymmetry  in  many  places. 
In  general,  it  may  be  said  that  all  intervals  are  smaller  on  the  nasal  side, 
larger  on  the  temporal  side. 

Pupil  and  lens  are  slightly  displaced  to  the  nasal  side;  in  the  pupil 
this  can  be  seen  even  in  the  living  eye;  the  shifting  of  the  lens  is  recog- 
nized by  the  fact  that  the  circumlental  space  (PI.  II,  i,  CI)  is  narrower 
on  the  nasal  than  on  the  temporal  side. 

The  form  of  the  ciliary  muscle  (Text  Fig.  i)  approaches  more  nearly 
the  type  of  that  in  hyperopia  on  the  nasal  side,  in  myopia  on  the 
temporal  side  (cf.  chap,  ix),  and,  corresponding  to  this,  there  are  slight 
differences  in  the  formation  of  the  iris  angle  and  the  position  of  the 
canals  of  Schlemm  in  relation  to  the  root  of  the  iris.  Most  striking, 
however,  is  the  difference  in  the  length  of  the  ciliary  muscle  (nasal 
about  5  mm,  temporal  6  mm).  As  a  result  the  retina  reaches  farther 
forward  on  the  nasal  than  on  the  temporal  side  (PI.  II,  i). 

The  emissaria  as  a  rule  are  longer  and  lie  farther  back  on  the  temporal 
than  on  the  nasal  side. 

Moreover,  the  upper  and  lower  halves  are  not  symmetrical,  yet  the 
differences  are  for  the  most  part  slight  and,  therefore,  not  conspicuous. 


B.     SPECIAL   ANxVrOMY   AND    HISTOLOGY    OF    THE 
EYEBALL 

CHAPTER  I.     SCLERA  (SCLEROTICA) 

The  outer  surface  of  the  sclera  has  a  dull-white  color.  A  very 
delicate  fine  connective  tissue  clings  to  it  and  forms  a  union  with  Tenon's 
capsule — that  layer  immediately  surrounding  the  eyeball  in  the  orbit. 
Only  rarely  does  the  outer  surface  of  the  sclera  contain  any  pigment; 
it  rather  comes  to  view  in  the  form  of  slate-gray  flecks  about  the  canals 
of  the  anterior  ciliary  vessels  (congenital  melanosis  of  the  sclera). 

The  inner  surface  of  the  sclera  appears  much  smoother  and  in  front  it 
has  an  almost  silky  luster;  posteriorly,  it  takes  on  an  increasingly  brown 
color  and  a  duller  appearance,  on  account  of  the  greater  number  of  the 
suprachorioidal  lamellae  which  cling  to  it  and  the  appearance  of  pigment 
cells  in  the  sclera  itself.  The  cut  surface  shows  a  tendonous  reflex  in 
places. 

The  thickness  of  the  sclera  varies  greatly  with  the  individual;  in 
youth  and  in  the  female  sex  it  is,  in  general,  thinner.  The  maximal 
scleral  thickness  (Text  Fig.  i)  is  at  the  posterior  pole  (i  mm  and  over; 
according  to  Stilling  [213],  it  varies  between  0.5  and  1.6  mm).  From 
here  the  thickness  gradually  decreases  to  0.4-0.6  mm  toward  the 
equator.  The  minimal  thickness  is  immediately  beneath  the  recti 
muscles  close  to  the  lines  of  insertion  of  their  tendons  (o .  3  mm) .  The 
tendons  show  a  varying  thickness  at  the  insertion  lines  but  in  many  cases 
they  are  as  thick  as  the  sclera  itself  (o .  3  mm)  so  that  immediately  in 
front  of  the  insertion  a  thickness  of  o .  6  mm  is  attained  and  held  to  the 
margin  of  the  cornea. 

At  its  margin  the  tendon  texture  is  usually  quite  sharply  set  off  from 
that  of  the  sclera.  One  observes  the  following  varieties  of  position  and 
direction  in  this  border  line:  (i)  The  border  runs  at  right  angles  to  the 
surface  of  the  sclera  just  at  the  place  where  the  interval  between  the 
sclera  and  tendon  disappears;  the  sclera  forms  a  step.  (2)  The  border 
runs  obliquely  from  in  front  and  without,  backward  and  inward,  i.e., 
the  innermost  layers  of  the  tendon  (those  lying  nearest  to  the  sclera) 
lie  farther  backward;  the  outer  ones  go  farther  forward  over  into  the 
scleral  tissue.  (3)  The  tendon,  as  a  whole,  courses  obliquely  inward 
into  the  sclera  and  disappears  farther  forward  in  the  scleral  tissue:  the 
tendon  takes  root,  so  to  say,  in  the  sclera. 


i8  ANATOMY  AND  HISTOLOGY  OF  THE  HUMAN  EYEBALL 

All  the  vessels  and  nerves  supplying  the  uvea  pass  through  the  sclera; 
they  are  only  loosely  imbedded  in  special  canals.  Following  an  older  con- 
ception of  the  lymph  circulation,  one  designates  these  canals  as  emissaria. 

The  emissaria  of  the  short  posterior  ciliary  arteries  course  in  very 
different  ways,  sometimes  straight,  sometimes  obliquely,  sometimes  bent 
at  an  angle;  the  inner  ends  often  lie  nearer  the  optic-nerve  entrance  than 
do  the  outer  ones. 

The  emissaria  of  the  long  posterior  ciliary  arteries,  of  the  ciliary  nerves 
and  of  the  vortex  veins  course  very  obliquely  from  without  and  behind^ 
inward  and  forward.  The  outer  (posterior)  portion  of  such  an  emis- 
sarium  courses  wholly  flat,  almost  parallel  to  the  outer  surface  of  the 
sclera  (PI.  Ill,  3)  and  the  outer  opening  of  the  emissarium  is  bordered 
in  front  by  a  sharp  semilunar  margin  and  passes  over  onto  the  outer  sur- 
face of  the  sclera  behind  in  a  furrow.  The  inner  end  of  the  emissaria 
is  more  steep,  has  the  sharp  semilunar  border  behind,  and  in  front  passes 
over  into  an  albeit  short  and  only  slightly  prominent  furrow.  In  longi- 
tudinal section  the  emissarium,  therefore,  forms  a  flat  bow  with  its 
concavity  inward. 

The  long  posterior  ciliary  arteries  are  accompanied  by  large  nerves; 
a  cross-section  of  a  given  emissarium  (PI.  II,  2)  shows,  therefore,  two 
rounded  canals,  close  together  and  separated  only  by  a  thin  vaginal  wall 
of  scleral  tissue.  The  artery  (A)  lies  in  the  one  canal,  the  nerve  (N)  in 
the  other;  both  are  fastened  to  the  wall  of  the  emissarium  by  loose 
connective  tissue.  According  to  Fuchs  (65),  the  length  of  this  emissarium 
is  3  to  7  mm.  Its  direction  is  strictly  meridional  and  horizontal.  The 
rest  of  the  ciliary  nerves  show  a  wholly  similar  relationship. 

The  emissaria  of  the  vortex  veins  (PI.  II,  3)  vary  more  from  the 
meridional  direction ;  their  inner  ends  are  farther  removed  from  the  verti- 
cal meridian  than  their  outer.  (For  the  position  of  the  latter  see  p.  9.) 
The  length  of  the  emissaria  is  usually  3  mm;  the  upper  temporal,  only, 
may  attain  a  length  of  4.6  mm  (Fuchs,  65).  The  cross-section  of  the 
emissarium  shows  a  broad  and  low  lumen  (PI.  II,  4);  the  thin  vein 
wall  lies  with  its  long  side  against  the  sclera  and  no  space  intervenes;  on 
the  short  side,  only,  is  some  loose  connective  tissue  interposed. 

The  emissaria  of  the  anterior  ciliary  arteries  course  much  less 
obliquely,  are  often  almost  perpendicular  to  the  surface,  and  often 
extremely  wide.  The  intrascleral  nerve  loops  described  by  Axenfeld  (10) 
can  be  observed  here  and  there  in  these  emissaria.  There  are  variations 
in  the  course  of  the  ordinary  ciliary  nerves,  and,  according  to  Fritz  (64), 
are  most  often  found  under  the  musciilus  rectus  superior,  but  do  not  by 
any  means  occur  in  all  eyes. 


THE  SCLERA  19 

The  affected  ciliary  nerve  is  especially  thick,  courses  in  a  normal  way 
from  behind  throughout  the  perichorioidal  space,  then  presses  into  the 
emissarium  to  the  outer  surface  of  the  sclera,  bends  sharply  about  there, 
and  goes  back  again  through  the  emissarium  into  the  interior  of  the  eye 
and  into  the  ciliary  body,  where  it  divides  up  like  the  others.  The  caps 
of  the  loop  are  made  especially  thick  by  a  deposition  of  nuclear-rich 
connective  tissue  and  may  even  contain  ganglion  cells  (Fritz,  64). 

Good  drawings  of  such  nerve  loops  have  been  pubhshed  by  Naito  (164),  Groenouw 
(78),  and  especially  by  Meller  (150).  Although  Axenfeld  and  Naito  make  mention  of 
no  blood-vessels,  I  hold,  with  Fritz,  that  anterior  ciliary  arteries  are  constantly  or 
very  frequently  associated  with  nerve  loops. 

When  the  nerve  loops  are  not  so  beautifully  cut  (longitudinally)  as  in  the  reported 
drawings,  they  often  give  beginners  very  considerable  difEculty  in  recognizing  them  as 
nerve  loops.  One  must  think  of  this  possibiUty  when  one  finds  the  continuity  of  the 
sclera  sharply  broken  in  the  region  of  the  posterior  border  of  the  ciliary  muscle  and 
this  place  filled  out  by  a  clear,  nuclear-rich  tissue  with  fibers  of  another  course.  The 
demonstration  of  medullary  sheaths  in  this  tissue  makes  the  diagnosis. 

The  structures  going  through  all  of  the  emissaria  (blood-vessels  or 
nerves)  are  for  the  most  part  bound  to  the  wall  of  the  emissarium  by 
loose  tissue  only.  Outwardly,  this  tissue  is  a  continuation  of  the  loose 
connective  tissue  which  unites  the  Tenon's  capsule  to  the  bulb  (PL  II, 
3,  Te);  inwardly,  it  is  a  continuation  of  the  suprachorioidea  and,  there- 
fore, characterized  by  pigmentation.  These  tissues  go  over  into  one 
another  in  the  middle  of  the  emissarium;  often  there  is  here  a  firmer 
fi.xation  to  the  wall,  especially  along  the  vortex  veins.  Yet  many  times 
pigment  cells  (chromatophores)  can  be  followed  throughout  the  entire 
length  of  the  emissaria  out  onto  the  surface  of  the  sclera. 

Corresponding  to  the  situation  of  this  tissue,  the  spaces  found  on  each 
side  of  the  sclera  (perichorioidal  and  Tenon's  spaces)  extend  into  the 
emissaria  and,  indeed,  at  the  ends  of  the  canals  these  spaces  between  the 
walls  of  the  canal  and  the  vessel  or  ner\-e  become  very  plain  and  visible 
without  any  special  help.  With  respect  to  the  middle  of  the  canal,  views 
vary.  According  to  the  older  conception  of  Schwalbe  (194),  the  peri- 
chorioidal and  Tenon's  space  are  lymph  spaces  and  the  emissaria  are  to 
be  looked  upon  as  communications  between  the  two.  It  was,  however, 
admitted,  even  by  the  supporters  of  this  view  (Fuchs,  65),  that  it  was 
difficult  or  impossible  to  inject  the  emissaria.  A  later  investigator 
(Langer,  136)  denies  any  communication  between  Tenon's  and  the  peri- 
chorioidal space,  and  conceives  of  the  spaces  found,  in  his  opinion,  at  the 
ends  of  the  emissaria,  as  well  as  the  perichorioidal  and  Tenon's  spaces  as 
a  whole,  as  articular  spaces.    The  oblique  direction  of  most  of  the  emissaria. 


ANATOMY  AND  HLSTOLOCiY  OF  THE  HUMAN  EYEBALL 


in  any  case,  has  the  effect  of  preventing  tearing  and  bending  in  the  motion 
of  the  eyeball  or  in  the  shifting  of  the  chorioidea  in  the  act  of  accom- 
modation. 

In  any  case,  simple  anatomic  study  shows  every  emissarium  to  Ije  a  locus  minoris 
resistentiae,  anci  this  comes  fundamentally  into  consideration  in  the  matter  of  the 
extension  of  malignant  tumors. 

The  union  of  the  sclera  with  the  optic  nerve  and  its  sheaths  will  be 
more  accurately  discussed  in  the  consideration  of  the  optic-nerve  entrance 
(chap.  viii). 

The  inner  surface  of  the  sclera  presents  a  shallow  furrow  close  to  its 
anterior  limit  (scleral  furrow,  or  sulcus  sclerae  inter  mis,  Text  Fig.  3, 

Sw-iH).  The  back  margin  of  this 
furrow  projects  a  little  forward  and 
somewhat  toward  the  interior  of  the 
eye,  and  is  known  as  the  scleral  spur 
or  better  as  the  scleral  roll  (PL  III, 
I,  Sw).     The  ciliary  body  is  inserted 


into  it  (here  one  finds  the  anterior 
insertion  ring  of  the  uvea).  The 
anterior  border  of  the  furrow  slopes 
very  gradually  over  onto  the  inner 
surface  of  the  cornea;  the  breadth  of 
the  furrow  is  about  0.75  mm.  On 
the  floor  of  the  furrow,  close  to 
its  posterior  margin,  lies  Schlemm's 
canal  (PI.  Ill,  i,  Sch);  the  rest  of 
the  depression  is  filled  out  by  the 
meshwork  of  the  iris  angle.  With 
respect  to  the  histology  of  this  por- 
tion, see  chap  iii. 

On  the  meridional  section  the  corneal  margin  of  the  sclera  in  the 
fresh  cadaver-eye  appears  to  the  naked  eye  to  be  a  pretty  sharp  line, 
because  the  white  of  the  sclera  stands  out  well  from  the  transparent 
cornea.  But  even  when  magnified  by  the  loupe  the  sharpness  of  this 
border  is  lost  in  part  and  entirely  so  in  the  fully  prepared  and  stained 
section,  cleared  and  mounted  in  balsam;  the  sclera  takes  on  a  somewhat 
denser  stain  than  does  the  cornea,  but  these  differences  merge  imper- 
ceptibly into  one  another.  One  inust,  therefore,  use  fresh  material  to 
study  the  position  and  course  of  the  corneoscleral  margin. 

In  horizontal  sections  the  border  line  is  then  seen  to  run  from  without 
inward  toward  the  ciliary  body  in  a  direction  about  parallel  to  the  optic 


Text  Fic.  3. — Tunica  fibrosa,  anterior  half. 
Viewed  from  within.     Magnification  2. 

Ac  equatorial  cut  surface,  Ac  white  stripes 
corresponding  to  the  course  of  the  long  posterior 
ciliary  arteries,  Su<  scleral  roll  (back  margin  of 
the  scleral  furrow),  iH  inner  corneal  margin 
(unbroken  line),  aH  outer  corneal  margin 
(dotted  line). 


THE  SCLEIL\  21 

axis.  However,  it  then  bends  about  a  little  along  the  inner  surface  to- 
ward the  axis  (PI.  I;  the  corneoscleral  border  is  indicated  by  a  punctate 
line).  The  floor  of  the  scleral  furrow  is  therefore  formed  of  scleral  tissue, 
as,  indeed,  preparations  of  the  inner  surface  also  show,  although  the 
corneoscleral  margin  is  soon  collinear  with  its  posterior  border. 

When,  on  the  other  hand,  one  studies  a  vertical  section,  a  very  oblique 
course  of  the  corneoscleral  border  is  shown,  so  that  its  outer  end  lies  i  mm 
or  more  nearer  the  optic  axis  than  does  the  inner  one.  (Concerning  the 
effect  which  this  has  upon  the  form  of  the  cornea,  consult  chap,  ii.) 

The  corneoscleral  border,  therefore,  meets  the  outer  surface  of  the 
sclera  at  an  acute  angle  (less  than  90°).  This  is  the  main  reason  why 
in  life  the  white  of  the  sclera  does  not  appear  sharply  set  off  from  the 
transparent  cornea;  a  very  narrow  transition  zone,  which  is,  neverthe- 
less, always  demonstrable  with  the  loupe,  is  present.  Moreover,  the 
circumstance  that  the  conjunctiva  gradually  goes  over  onto  the  trans- 
parent cornea  contributes  to  this  obscuration  of  the  corneal  margin. 

Above  and  below,  therefore,  this  extension  of  the  sclera  over  onto  the 
cornea  is  much  more  outspoken  than  along  the  nasal  and  temporal  margins; 
indistinctness  of  the  border  is  correspondingly  more  marked.  Owing  to  the 
fact  that  the  upper  margin  of  the  cornea  is  the  one  of  preference  for  opera- 
tive procedures,  these  relations  become  of  great  imporance  for  operative 
technique.  

The  sclera  as  a  whole  shows  a  very  uniform  structure,  but  one  can 
always  make  out  certain  modifications  of  the  structure  toward  the  outer 
surface  and — in  this  by  no  means  strict  sense  of  the  word — one  can 
speak  of  various  layers.  These  layers  from  without  inward,  are:  (i)  the 
episcleral  tissue;    (2)  the  sclera  proper;    (3)  the  lamina  fusca  sclcrae. 

I.     The  Episcleral  Tissue 

This  is  of  a  looser  structure  than  the  sclera  proper,  its  bundles  are 
more  delicate,  more  tortuous,  and  course  in  varying  directions.  Out- 
wardly, it  extends  over  into  the  loose  tissue  which  fills  out  Tenon's  space. 
Toward  the  inner  side  its  bundles  are  firmer,  thicker,  and  the  mattressing 
is  more  close.  In  this  way  it  graduates  over  into  the  tissue  of  the  sclera 
proper.  It  is  especially  characterized  by  relatively  numerous  vessels,  and 
so  is  easily  differentiated  from  the  vesselless  tissue  which  fills  out  Tenon's 
space,  as  well  as  from  the  tissue  of  the  sclera,  which  is  scanty  in  vessels. 
Therefore  one  makes  out  the  episcleral  tissue  best  in  much  inflamed  eyes, 
where  the  individual  tiny  vessels  are  filled  full  of  blood. 

Behind  the  insertion  line  of  the  recti  muscles,  the  episcleral  tissue 
forms  only  a  very  thin  layer  with  a  very  loose  net  of  vessels.     In  front 


22  ANATOMY  AM)  lIIsr()I.OG\'  OF  THE  HUMAN   KVKBALL 

of  this  insertion  line  it  is  much  more  strongly  developed  and  rich  in 
vessels,  since  the  eye  muscle-tendons  also  add  a  similar  tissue — a  thicker 
layer,  with  larger  blood-vessels  (perimysium).  This  tissue  continues, 
ecjually  thick,  forward  into  the  sulcus  sclerae  externus,  and  here  goes  over 
into  the  limbus  corneae  (PI.  I,  Es).  A  large  number  of  thick  elastic 
tissue  fibers  are  mixed  with  this  tissue. 

The  vessels  in  the  back  part  of  the  episcleral  tissue  are  branches  of  the 
posterior  ciliary  artery,  those  in  the  fore  part  of  the  tissue,  of  the  anterior. 
These  branches  form  a  network  in  the  usual  way,  that  is  to  say,  one 
artery  is  accompanied  by  two  veins;  the  meshes  of  this  net  are  very  wide 
in  the  back  part  and  first  become  increasingly  narrow  in  front  of  the  recti 
muscles  and  toward  the  border  of  the  cornea.  A  thicker  capillary  and 
venous  net  exists  only  in  this  anterior  zone  of  the  sclera;  a  marked  filling 
of  this  net  produces  the  so-called  ciliary  injection. 

2.      The  Sclera  Proper 

This  is  a  dense  fibrous  tissue  in  which  the  bundles  cross  in  the  most 
varied  directions  and  often  divide  at  sharp  angles.  At  least  one  obtains 
this  impression  in  surface-sections.  The  cross-section  (at  right  angles  to 
the  surface)  (PI.  II,  5)  shows,  on  the  other  hand,  long  band-like  stripes 
(apparently  representing  bundles  which  have  been  cut  in  their  long 
axis),  and  short  oval  or  lance-formed  fields  (apparently  cross-sections 
of  bundles),  and,  if  one  bases  his  studies  of  the  course  of  the  fibers  in 
the  sclera  solely  upon  such  cross-sections,  one  is  very  easily  led  to  the 
view  that  there  are  two  main  directions  of  the  scleral  bundles,  meridional 
and  equatorial. 

Indeed,  there  are  places  in  which  equatorial  bundles  are  solely  or 
mainly  present.  The  immediate  neighborhood  of  the  optic  nerve  is  such 
a  place.  A  second  is  the  scleral  roll  and  its  immediate  neighborhood. 
However,  in  the  remaining  portions  of  the  scleral  the  bundles  possess  a 
most  varying  direction. 

I  think  that  one  obtains  the  best  conception  of  the  structure  of  the 
sclera  from  a  consideration  of  the  inner  surface.  Here,  even  though  one 
is  not  able  to  follow  the  course  of  the  individual  bundles,  one  can  recog- 
nize the  prevailing  direction  from  the  silky  reflex  of  the  inner  surface,  or 
bundles  of  a  certain  direction  can  be  made  out,  to  some  extent. 

Then  one  can  recognize  an  equatorial  fibrillation  (concentric  to 
the  border  of  the  cornea)  just  along  the  posterior  edge  of  the  scleral 
furrow;  farther  back  the  fibers  form  more  marked  loops  with  their 
convexities  turned  backward,  and  in  this  way  they  gradually  go  over 
into  meridional  bundles.  If  one  conceives  of  these  loops  as  being  given 
off  from  all  points  of  the  corneoscleral  border,  one  realizes  that  a  crossing 


THE  SCLERA  23 

with  a  prevailing  circular  course  in  front  and  a  meridional  one  behind 
comes  about. 

In  the  body  of  the  sclera  one  cannot,  of  course,  follow  the  bundles  in 
this  way,  but  it  is  very  easy  to  believe  that  a  similar  principle  rules  here, 
i.e.,  all  the  bundles  form  loops,  and  whether  they  appear  as  meridional, 
oblique,  or  equatorial  bundles  depends  solely  upon  the  way  in  which 
the  loop  is  sectioned.  I  do  not  trust  myself  to  state  anything  more 
definitely  concerning  the  scleral  fibrillation,  despite  the  thorough  studies 
which  Ischreyt  (in)  has  made  concerning  them. 

The  bundles  are  quite  delicate  in  the  anterior  section  and  have  sharp 
borders;  in  the  posterior  section  one  sees  considerably  larger  bundles 
with  subdivisions  which  do  not  have  exactly  the  same  direction  and  are 
not  sharply  separated  from  one  another.  In  short,  the  structure  of  the 
sclera,  which  is  difficult  enough  to  make  out  anywhere,  becomes  more  and 
more  complicated  as  one  proceeds  backward. 

On  surface  view  the  individual  scleral  fiber-bundle  shows  a  fine  parallel 
striation.  The  dimension  of  the  bundle  in  the  direction  of  this  striation 
is  spoken  of  as  the  length  of  the  bundle,  the  dimension  at  right  angles  to 
the  striation  and  parallel  to  the  surface  of  the  sclera,  as  the  breadth, 
and  the  dimension  at  right  angles  to  the  surface  of  the  sclera  as  the 
thickness  of  the  bundle. 

We  must  look  upon  the  length  of  the  bundle  as  an  unconditionally 
great  one;  in  any  case  one  cannot  measure  it  microscopically,  because 
the  bundles  are  always  cut  off.  In  thickness,  the  bundle  measures  10  to 
16  mu  in  the  anterior  segment  (when  pure  cross-sections  are  studied); 
the  breadth  varies  according  to  the  locality.  In  general,  I  hold  this  to 
be  100  to  140  mu  in  the  anterior  segment,  and  the  relation  of  the  thick- 
ness to  the  breadth  to  be  something  like  i  to  10  or  12;  the  cross- 
section,  therefore,  has  a  somewhat  elongated  lance-form.  The  circular 
bundles  of  the  scleral  roll  are  much  narrower  (30  to  50  mu),  and  the 
relation  of  the  thickness  to  the  breadth  is  i  to  3.  The  cross-section 
is  oval  or  has  the  form  of  a  myrtle  leaf.  In  the  posterior  section  one  is 
so  often  in  doubt  where  the  border  of  the  bundle  lies  that  one  cannot 
accurately  measure  it. 

The  bundles,  almost  throughout,  course  parallel  to  the  surface;  even 
when  the  intermixing  of  the  bundles  shows  slight  variations  from  this 
direction,  it  is  scarcely  noticeable.  Only  along  the  emissaria  are  bundles 
found  which  have  a  course  that  is  oblique,  or,  indeed,  at  right  angles 
to  the  surface;  it  is  the  narrow  sides  of  the  individual  canals  which  are 
flanked  by  such  bundles. 

The  funiculus  sderoticae  described  by  Hanover  (87)  (according  to  him,  a  remnant 
of  the  fetal  cleft)  appears  to  be  only  such  a  bundle  flanking  a  posterior  ciliary  artery. 


24  ANATOMY  AND  HISTOLOGY  OF  THE  HUMAN  EYEBALL 

The  individual  bundle  consists  of  line  collagenous  fibrillae,  coursing 
parallel  to  each  other  and  to  the  length  of  the  bundle;  the  fine  striation 
visible  in  surface-sections  comes  from  this  and  gives  the  direction  to  the 
entire  bundle.  The  longitudinal  section  shows  either  a  similar  striation 
or  a  more  homogeneous  appearance  (PI.  II,  5,  /).  Cross-sections  do  not 
show  the  individual  fibrillae  but  only  groups  of  them;  the  entire  cross- 
section  of  the  bundle  is  divided  into  smaller  angular  fields  in  this  way, 
giving  it  a  peculiar  appearance  which  I  call  the  cross-section  marking  for 
short  (PI.  II,  5,  q). 

Aside  from  the  collagenous  fibers,  ordinary  elastic  ones,  i.e.,  fibers 
staining  with  orcein,  are  visible.  They  occupy  the  periphery  of  the 
bundle  and  only  a  few  extend  into  the  interior  between  the  groups  of 
fibrillae.  As  a  whole  the  elastic  fibers  course  parallel  to  the  direction  of 
the  bundles.  They  are  very  much  more  delicate  than  in  other  tissues, 
and,  therefore,  are  to  be  made  out  in  longitudinal  section  and  in  surface 
preparations  only  by  strong  staining.  They  are  more  easily  seen  in 
cross-section. 

Between  the  bundles,  especially  in  the  angles  formed  by  the  crossing 
of  the  bundles,  lie  the  fixed  cells  of  the  scleral  tissue;  they  are  also  visible 
in  the  posterior  portions,  on  account  of  the  incomplete  separation  of  the 
bundles  from  one  another. 

After  Held's  stain  these  cells  come  out  in  surface-sections  as  membra- 
nous structures,  with  an  extremely  thin  and  only  very  faintly  stained  body 
going  over  into  finer  and  broader  extensions.  The  fine  extensions  seem 
to  course  in  the  direction  of  the  collagenous  fibrillae,  the  broader  ones 
across  these.  The  cells  are  united  with  one  another  by  means  of  these 
processes;  they  form  a  syncytium,  yet  I  think  that  this  is  not  as  com- 
pletely closed  on  all  sides  as  in  the  cornea.  The  cell-nuclei  are  very 
irregular  in  form  owing  to  the  position  of  the  cell;  for  the  most  part, 
however,  they  are  longish,  have  a  fine  chromatin  mesh  and  i  to  3  very 
small  nucleoli.  The  long-strung-out  spindle-form  cells,  which  are  not 
infrequently  seen  in  surface-sections,  likewise  appear  to  be  provided  with 
very  long  nuclei  lying  mainly  in  the  angles  between  the  bundles;  these 
may  in  part  be  only  side  views  of  fiat  cells. 

With  certain  stains  the  scleral  bundles  seem  to  me  to  possess  border 
membranes  in  the  anterior  segment;  for  example,  beside  the  elastic 
fibers  the  orcein  stain  shows  an  extremely  fine,  weak,  brownish  layer 
surrounding  the  bundle.  These  membranes  are  connected  with  the 
cells  in  any  case;  according  to  Pes  (171),  they  connect  the  elastic  fibers 
with  the  cell-processes. 

The  blood-vessels  of  the  sclera  fall  into  two  groups.     The  one  makes 


THE  SCLERA  25 

use  of  the  sclera  only  for  passage,  and  subdivides  into  capillaries  in  other 
tissues;  the  vessels  of  the  uvea  belong  in  this  group.  The  points  of 
entrance  for  these  have  already  been  described  as  the  emissaria.  There 
are,  however,  a  large  number  of  smaller  vessels  in  the  sclera,  which  do  not, 
properly,  supply  this  membrane  itself,  e.g.,  in  the  neighborhood  of  the 
optic  nerve.  Here  several  branches  of  the  short  posterior  ciliary  arteries 
form  an  anastomotic  ring  about  the  optic  nerve  (the  circle  of  Zinn  or 
Holler's  vascular  circle,  or  the  circidus  arteriosus  nervi  optici  (Leber), 
which,  as  the  name  suggests,  serves  for  the  supply  of  the  optic  nerve, 
especially  the  lamina  cribrosa.  Moreover,  in  the  most  anterior  portion  of 
the  sclera  just  behind  and  outside  Schlemm's  canal  relatively  numerous 
vessels  are  present,  for  the  anterior  ciliary  veins  pass  through  here. 

How  insignificant  the  second  group  is,  i.e.,  how  few  vessels  the  sclera 
possesses  in  and  of  itself,  comes  out  best  in  the  region  of  the  equator: 
here  and  there  a  capillary  lumen  is  visible,  but  in  a  cursory  inspection  this 
tissue  appears  entirely  devoid  of  vessels. 

The  nerves  of  the  sclera  are  branches  of  the  ciliary  nerves ;  they  branch 
off  in  the  perichorioidal  space  and  broaden  out  (especially  in  the  inner 
two-thirds  of  the  sclera)  and  form  a  trabeculum  similar  to  that  in  the 
stroma  of  the  cornea. 

According  to  Smirnow  (209)  and  Agababow  (6),  there  are  three 
sorts  of  endings  in  the  scleral  tissue:  free  sensory  endings,  trophic  ones 
in  connective-tissue  cells,  vasomotor  ones  in  vessels.  Finally,  Agababow 
describes  a  rich  nerve-fiber  net  in  the  lamina  fusca.  In  any  case,  most 
of  these  findings  are  not  marked  in  human  eyes;  only  sensory  endings 
have  been  proven  for  the  human  (by  Smirnow). 

In  general,  the  anterior  part  of  the  sclera  is,  furthermore,  perforated 
by  numerous  finer  nerves  which  supply  the  cornea  (Fritz,  64). 

The  histologic  relations  of  the  tendon  insertions  are  best  studied  in 
sections  which  go  at  right  angles  to  the  line  of  insertion,  i.e.,  meridional 
sections  in  the  case  of  the  recti  muscles.  The  tendon  itself  (PI.  I,  Mr) 
consists  of  a  mass  of  exactly  longitudinal  coursing  bundles  of  collagenous 
fibrillae  supported  by  thick  elastic  fibers.  Aside  from  the  difference  in  the 
size  of  these  fibers,  it  is  made  up  of  the  same  tissue  as  the  sclera;  the 
texture  only  is  different :  in  the  tendon  all  the  bundles  are  parallel,  hence 
the  peculiar  sUky  reflex ;  in  the  sclera  itself  the  bundles  are  much  crossed, 
hence  the  duU-white  appearance.  The  tendon  in  this  way  goes  directly 
over  into  the  tissue  of  the  sclera;  the  bundles  of  the  tendon  spread  apart 
and  interweave  with  the  cross  and  oblique  bundles.  The  tendon-bundles 
go  over  into  the  direction  of  the  scleral  bundles  concerned  and  only  the 
cessation  of  the  thick  elastic  fibers  marks  the  border  line. 


26  ANATOMY  AND  HISTOLOGY  OF  THE  HUMAN  EYEBALL 

3.     Lamina  Jiisca  sclerae 

Near  the  inner  surface  the  connective-tissue  bundles  become  smaller 
and  flatter,  the  elastic  fibers  more  numerous  and  thicker;  branched  pig- 
ment cells  (chromatophores)  appear  here  and  there  in  the  interspaces 
of  the  tissue.  Finally,  along  the  inner  surface  itself  the  whole  is  closed 
off  by  an  endothelial  layer.  This  insignificant  modification  of  the  scleral 
tissue  on  its  inner  surface  is  the  lamina  fusca.'- 

The  name  of  the  lamina  fusca  is  given  to  it  by  the  brown  color  which 
the  pigment  cells  lend  to  the  inner  surface  of  the  sclera.  It  cannot  be 
isolated,  for  it  goes  continuously  over  into  the  tissue  of  the  sclera  itself. 
What  one  brushes  away  is  only  the  lamellae  of  the  suprachorioidea  clinging 
to  the  lamina  fusca. 

In  a  histologic  sense  the  lamina  fusca  forms  the  transition  over  into 
suprachorioidea.  The  new  tissue  elements  appearing  in  it  are  identical 
with  those  of  the  suprachorioidea  and  will  be  more  accurately  described 
under  that  heading. 

The  pigmentation  of  the  lamina  fusca  is  not  uniform;  anteriorly, 
immediately  behind  the  scleral  roll  (Text  Fig.  3,  Sw),  it  is  as  good  as 
colorless,  and,  moreover,  farther  back  the  brown  color  only  appears  in 
flecks.  Farther  back  it  usually  becomes  more  intense  along  the  larger 
ciliary  nerves;  the  pigmentation  is  weaker  along  the  larger  nerves  and 
in  particular  corresponding  to  those  bundles  which,  with  their  accompany- 
ing nerves,  form  the  long  posterior  ciliary  arteries;  for  this  reason  two 
broad  white  streaks  appear  especially  prominent  in  the  horizontal 
meridian  along  the  inner  surface  of  the  sclera  (Text  Fig.  3,  Ac). 


CHAPTER  n.     THE  CORNEA 

The  cornea  has  the  form  of  a  strongly  curved  meniscus,  to  which  in 
and  of  itself  one  must  ascribe  a  weak  refracting  power,  because  it  is  thinner 
in  the  center  (0.8  mm)  than  it  is  at  the  edge  (i  mm,  or  slightly  more). 

The  measurement  of  the  thickness  of  the  cornea  is  difficult,  because  the  stroma 
swells  up  easily  in  water;  sections  hardened  in  Mueller's  fluid  also  usually  show  the 
cornea  in  a  swollen  condition.  The  swelling  is  mainly  inward;  a  marked  tumifaction 
develops  at  the  edge  of  Descemet's  membrane  (see  PI.  I),  and  the  entire  inner 
surface  becomes  wavy.  In  this  respect  formalin-alcohol  hardening  conserves  the 
cornea  better. 


'  The  meaning  of  this  term  is  not  the  same  to  all  authors;  by  it  many  understand  that  part  of  the 
suprachorioidal  lamellae  which  remains  clinging  to  the  sclera  after  detachment  of  the  uveal  tract,  others 
probably,  the  whole  suprachorioidea  as  well.  It  is  perhaps  best  to  allow  this  expression  to  drop  entirely 
into  disuse. 


THE  CORNEA  27 

The  radius  of  curvature  of  the  anterior  surface  is  7 .  84  mm  on  the 
average  (computed  from  the  43  .03  D.  of  average  corneal  refraction  given 
by  Steiger,  211). 

This  radius  of  curvature  only  holds  true,  however,  for  the  central 
third  of  the  cornea,  the  so-called  optical  zone,  which  is  curved  almost 
exactly  like  a  sphere;  the  peripheral  parts  are  notably  flattened  and  more 
so  upon  the  nasal  than  upon  the  temporal  side.  Aside  from  this  there 
usually  exists  a  certain  degree  of  meridional  asymmetry,  i.e.,  the  vertical 
meridian  is  somewhat  more  strongly  curved  than  the  horizontal  one. 

We  are  very  little  informed  concerning  the  curvature  of  the  posterior 
surface.  Merkel  and  Kallius  (151)  hold  it  to  have  the  curvature  of  a 
sphere  and  the  central  portion  to  be  concentric  with  the  anterior  surface. 
According  to  this,  its  radius  of  curvature  would  be  something  like  7  mm. 
Tscherning  (227)  has  measured  it  ophthalmometrically  in  the  living  and 
has  found  very  great  differences  in  the  curvature  of  the  anterior  and 
posterior  surfaces.  He  holds  the  radius  of  curvature  for  the  posterior 
surface  to  be  6. 22  mm. 

Viewed  from  the  front,  the  cornea  appears  weakly  elliptical  with  the 
longer  axis  horizontal.  According  to  Priestley  Smith  (174),  the  hori- 
zontal diameter  of  the  cornea  varies  between  the  extremes  of  10.5  and 
13.5  mm,  but  usually  is  between  11  and  12  mm.  The  average  for  eyes 
of  all  ages  is  1 1 . 6  mm:  for  males  1 1 . 65,  for  females  1 1 .  54.  The  vertical 
diameter  is  some  i  mm  less. 

Looked  at  from  behind  (within),  the  cornea  appears  completely 
circular  with  a  diameter  close  to  the  long  diameter  of  the  anterior  surface. 
This  is  due  to  the  form  of  the  corneoscleral  border  discussed  above. 
These  relations  come  out  very  beautifully  if  one  looks  at  the  anterior 
half  of  the  tunica  fibrosa  after  the  removal  of  the  other  membranes  and  the 
contents  of  the  bulb  (Text  Fig.  3) ;  one  then  sees  the  oval  contour  of  the 
anterior  surface  (aH)  outlined  in  the  circular  contour  of  the  posterior 
surface  (iH). 

A  plane  going  through  the  outer  visible  border  of  the  cornea  is  called 
the  base  of  the  cornea,  and  the  distance  between  it  and  the  center  of 
the  cornea  the  height  of  the  cornea;  this  amounts  to  about  2.6mm. 
It  increases  with  the  curvature  of  the  cornea  and  also  with  the  size  of  the 
cornea,  for  a  larger  segment  of  a  given  sphere  is  higher  than  a  smaller 
one.  Large  corneae  are,  therefore,  easily  held  to  be  more  sharply 
curved,  because  they  are  especially  high. 

Looked  at  from  within,  the  cornea  seems  deepened,  just  as  it  appears 
set  forward  when  looked  at  from  without.  The  margin  of  this  con- 
cavity shows  a  very  weak  rounding  which,  so  to  speak,  is  the  negative 


28  ANATOMY  AND  HISTOLOGY  OF    THE  HUMAN  EYEBALL 

of  the  sitlcus  sclerae  extcrniis  and  lies  along  the  inner  border  of  the  sulcus 
sclcrac  intern  us.  The  configuration  of  the  sinus  angle  (cf.  chap,  xiv), 
so  important  for  pathology,  is,  therefore,  in  part  conditioned  by  these 
relations. 

Apart  from  the  indistinctness  of  the  marginal  portions,  the  cornea 
everywhere  has  a  uniform  transparency,  at  least  in  middle  life.  These 
two  zones  concerning  which  we  now  speak,  cannot,  therefore,  be  macro- 
scopically  separated  from  one  another  in  life  and  are  only  characterized 
by  their  microscopic  structure.  Bowman's  membrane,  in  particular, 
does  not  extend  over  the  entire  cornea;  it  ceases  about  i  mm  from  the 
border  (PI.  I,  b)  and  the  remaining  marginal  portion  of  the  cornea,  which, 
too,  is  characterized  by  further  anatomic  peculiarities,  is  known  as  the 
limbus  corneae. 

The  limbus  is  therefore  a  zone  of  about  i  mm  width  limited  centrally 
by  the  margin  of  the  Bowman's  membrane  and  peripherally  by  the 
corneoscleral  border,  and,  properly  speaking,  includes  only  the  anterior 
layers  of  the  cornea.  For  the  purposes  of  description,  solely,  we  think 
of  the  limbus  as  bordered  by  a  plane  going  through  the  depths  of  the 
cornea  from  the  border  of  Bowman's  membrane  to  the  border  of 
Descemet's  membrane. 

The  remainder  forms  the  cornea  proper,  which  is,  therefore,  char- 
acterized anatomically  by  two  basal  membranes.  I  begin  with  the  descrip- 
tion of  the  cornea  proper,  because  it  is  then  easier  to  describe  the  limbus 
as  the  transition  zone  between  the  sclera  and  cornea. 

a)    The  Cornea  Proper 

In  this  portion  one  distinguishes  5  layers  from  before  backward  (with- 
out inward):  (i)  Epithehum;  (2)  Bowman's  membrane;  (3)  Stroma 
corneae;    (4)  Descemet's  membrane;    (5)  Endothelium. 

I.      THE   EPITHELIUM   OF   THE    CORNEA 
(PL  II,  6,  Ep) 

Over  the  greater  part  of  the  cornea  this  has  a  uniform  thickness  of 
37  to  58  mu;  a  slight  increase  in  thickness  is  first  noted  near  the  edge 
of  Bowman's  membrane.  It  possesses  two  wholly  smooth  border  sur- 
faces, an  anterior,  formed  by  the  anterior  surface  of  the  cornea,  and  a 
posterior,  which  is  in  contact  with  Bowman's  membrane;  the  cross- 
section,  therefore,  shows  two  exactly  concentric  contours,  exactly  parallel 
when  small  stretches  are  looked  at  by  high  power. 

In  the  great  regularity  of  the  anterior  surface  (the  anterior  contour 
in  the  microscopic  preparation)  the  epithelium  of  the  cornea  probably 


THE  CORNEA  29 

surpasses  the  epithelium  of  all  other  parts  of  the  body,  and  this  regu- 
larity has  a  connection  with  the  function  of  the  anterior  surface  as  a 
part  of  the  optic  system.  The  great  brilliancy  of  the  cornea,  the 
regular  reflex  of  its  anterior  surface,  is  only  present  when  the  epithelium 
is  completely  intact. 

Unfortunately,  one  rarely  sees  this  normal  structure  of  the  epithelium 
in  the  microscopic  preparation,  since  it  is  easily  subject  to  mechanical 
lesions  and  post-mortem  changes,  such  as  drying,  and  also  because  the 
hardening  fluids  act  upon  the  epithelium  before  they  do  upon  the 
deeper  portions  and  to  a  greater  extent. 

The  corneal  epithelium  is  a  stratified  pavement  epithelium  of  5  to  6 
layers  of  cells. 

The  deepest  layer  (basal  or  foot-cells,  b)  lies  directly  over  Bowman's 
membrane,  and  consists  of  cylindrical  cells  of  some  18  mu  in  height  and 
10  mu  in  breadth;  each  cell  turns  an  absolutely  flat  surface  (foot)  toward 
Bowman's  membrane,  and  a  rounded  end  (head)  to  the  succeeding  cell- 
layer.  The  nucleus  is  slightly  oval  (5X7  mu)  and  lies  with  its  long  axis 
at  right  angles  to  the  surface  of  the  cornea. 

As  shown  by  these  figures,  the  basal  cells  of  the  corneal  epithelium  are 
characterized  by  a  considerable  size  and  rich  protoplasm.  The  proto- 
plasm is  for  the  most  part  somewhat  lighter  than  that  of  the  succeeding 
cell-laj-ers.  Yet  places  are  found  in  the  basal  cell-layer  in  which  the 
protoplasm  stains  darker;  these  cells  are  longer,  as  well,  and  have 
concave  sides;  their  nucleus  is  displaced  into  the  head  and  is  obliquely 
oval.  These  are  the  cells  seen  passing  up  into  the  next  layer  (H. 
Virchow,  234)  (PL  II,  6,  the  cell  next  to  the  last  on  the  right). 

According  to  v.  Ebner  (48),  the  basal  cells  are  positive,  monaxial, 
and  double  refracting,  with  the  optic  axis  perpendicular  to  the  surface. 

The  second  layer  ("wing"  cells)  consists  of  polyhedral  elements  with 
convex  anterior  surfaces  and  concave  posterior  ones;  the  edges  between 
these  concave  surfaces  are  more  or  less  drawn  out  into  the  shape  of  wings. 
The  long  axis  of  the  nucleus  is  parallel  to  the  surface  of  the  cornea,  and 
the  protoplasm  is  of  a  darker  color  than  that  of  the  basal  cells.  The  cells 
of  the  middle  layers  are  not  double  refracting  (v.  Ebner). 

While  the  cells  of  the  second  layer  are  of  about  equal  size  in  all  three 
dimensions,  a  beginning  flattening  is  apparent  in  the  third  layer,  i.e.,  the 
cells  are  larger  parallel  to  the  surface  and  the  nuclei  are  more  oval.  The 
transition  to  the  very  flat  cells  of  the  surface  layer  (0)  (i.e.,  5th  to  6th)  is 
carried  out  in  this  way.  These  cells  are  very  large  in  surface  expanse  (up 
to  46  mu)  but  are  very  thin,  in  general,  and  only  measure  some  4  mu  in 
the  region  of  the  nucleus.     This  thickening  projects  backward  toward  the 


30  ANATOMY  AND  HISTOLOGY  OF  THE  HUMAN  KYKHALL 

deeper  layers,  while  the  anterior  surface  is  wholly  flat.    The  complete  regu- 
larity of  the  anterior  surface  of  the  epithelium  comes  about  in  this  way. 

The  nuclei  of  the  surface  cells  are  likewise  much  flattened  (2.5X12 
mu),  stain  less  densely  than  those  of  the  deeper  layers,  but  show  normal 
structure  and  no  indications  of  dissolution,  therefore  no  keratosis,  as  in 
the  epidermis.  According  to  v.  Ebner  (23),  the  surface  cells  are  negative, 
monaxial,  double  refracting,  with  the  optic  axis  perpendicular  to  the 
surface. 

The  union  of  the  cells  with  one  another  is  formed  by  means  of  cell- 
bridges,  as  in  the  epidermis.  The  cell-outlines  acquire  a  similarity  to 
ladders,  because  these  bridges  shrink  easily  and  become  drawn  out  into 
threads.  The  totality  of  the  space  between  these  bridges  forms  the 
intercellular  canal  or  lymph  system;  it  can  be  injected  with  fluid  and  is 
often  found  widened  in  pathologic  conditions  (e.g.,  in  glaucoma). 

The  intercellular  spaces  are  plainest  between  the  basal  cells;  toward 
the  surface  they  gradually  disappear.  They  do  not  come  out  plainly 
after  all  hardening  fluids;  with  Mueller's  fluid  one  usually  gets  simple 
linear  cell  contours ;  formalin-alcohol,  and  other  similar  reagents,  bring  out 
the  cell-bridges  better;  one  sees  them  best,  however,  in  pathologic  cases. 

Leucocytes  (wandering  cells)  are  quite  often  found  in  this  system 
of  spaces.  They  are  characterized  by  their  very  heavy  staining,  con- 
stricted, or  fragmented  nuclei.  Under  normal  conditions  they  are  prob- 
ably only  to  be  found  between  the  bases  of  the  foot-cells  just  in  front  of 
Bowman's  membrane  (PI.  II,  6;  a  little  to  the  left  of  the  center).  Under 
pathologic  conditions  they  are  much  more  numerous  and  extend  farther 
into  the  epithelial  layers.  Concerning  the  nerves  of  the  epithelium  see 
P-  35- 

2.     bowman's  membrane  {Lamina  elaslica  anterior) 
(PI.  II,  6,  B) 

This  membrane  has  a  uniform  thickness  of  10  to  16  mu  throughout 
almost  its  entire  expanse,  and,  with  the  exception  of  its  pores,  is  wholly 
structureless,  showing  neither  cell-nuclei  nor  fibrillation.  Its  anterior 
surface  is  sharply  set  off  from  the  epithelium  and  has  a  curvature  con- 
centric with  the  anterior  epithelial  surface.  It  is  absolutely  smooth;  I 
have  not  seen  the  fine  serrations  described  by  H.  Virchow  (234)  in  any 
of  my  preparations. 

When,  therefore,  a  larger  expanse  of  epithelium  has  been  lost,  as  after  a  slight 
burn,  the  recognition  of  such  a  loss  of  substance  is  often  very  difficult;  the  curvature 
of  the  cornea  appears  unchanged  but,  when  one  gets  the  reflection  of  the  image  of  the 
mirror  from  the  margin  of  the  loss  of  substance,  the  step  caused  by  the  desquamation 
of  the  epithelium  is  recognized. 


THE  CORNEA  31 

On  section,  the  posterior  surface  of  Bowman's  membrane  does  not 
show  so  sharp  a  contour  as  the  anterior,  for  it  merges  with  the  most 
superficial  lamellae  of  the  corneal  stroma.  Furthermore,  it  is  not  possible 
to  detach  Bowman's  membrane  from  the  stroma. 

In  its  staining  reaction  it  agrees  fully  with  the  stroma  lamellae,  and 
only  in  its  homogeneity  does  it  differ  from  the  fibrillar  appearing  lamellae. 
Bowman's  membrane  is  therefore  to  be  looked  upon  as  a  specially 
modified  superficial  stroma  layer. 

The  sole  details  of  structure  which  one  occasionally  but  by  no  means 
constantly  recognizes  in  cross-section  of  Bowman's  membrane  are  the 
pores  for  the  rami  perforantes  of  the  corneal  nerves.  These  are  not 
characterized  by  a  different  staining  but  solely  by  their  extremely  deli- 
cate contours.  Corresponding  to  each  pore  there  are  two  contours, 
visible  only  by  very  sharp  focusing;  they  are  usually  parallel  to  one 
another,  although  not  coursing  exactly  at  right  angles  to  the  surface. 

In  most  eyes  the  periphery  of  Bowman's  membrane  is  sharpened 
from  behind  forward. 

3.     THE  Substantia  propria  OF  THE  cornea  (Stroma  corneae) 

{Substantia  propria  corneae) 

(PI.  II,  6,  9,  C) 

This  forms  the  main  mass  of  the  cornea,  comprising  about  90  per  cent 
of  the  entire  thickness. 

No  tissue  of  the  eye  has  been  more  thoroughly  and  carefully  studied 
than  the  corneal  stroma,  and  yet  we  are  far  from  a  full  understanding  of 
its  structure.  It  is  not  my  task  to  go  into  all  the  disputed  questions  and 
small  details;  those  who  wish  to  familiarize  themselves  with  these  things 
should  consult  H.  Virchow's  (234)  exhaustive  and  critical  presentation. 
The  following  description  will  be  limited  to  the  most  important  struc- 
tural details,  especially  to  those  which  one  can  see  in  sections  after  ordi- 
nary stains,  and  these  alone  can  serve  as  the  basis  for  consideration  of 
pathologic  conditions. 

The  circumstance  that  the  spaces  in  the  tissue  in  which  the  nuclei 
lie  do  not  unite  with  one  another  and  course,  throughout,  parallel  to  the 
surface  is  the  most  striking  thing  seen  in  a  section  perpendicular  to 
the  surface ;  this  is  characteristic  of  the  corneal  stroma  and  differentiates 
it  from  the  sclera,  as  well  as  from  pathologic  scar-tissue.  This  picture 
comes  out  in  any  section,  independent  of  its  direction,  provided  it  is  per- 
pendicular to  the  surface.  From  this  alone  one  can  draw  the  conclusion 
that  the  stroma  corneae  is  made  up  of  lamellae  parallel  to  the  surface. 


32  ANATOMY  AND  HISTOLOGY  OF  THE  HUMAN  EYEBALL 

That  which  lies  between  two  spaces  is  not,  however,  a  simple  lamella; 
this  strip  of  tissue  is  not  a  unit  of  structure  but  a  still  larger  number  of 
finer  outlines  parallel  to  the  surface  can  be  made  out  (PL  II,  9),  i.e.,  it 
is  made  up  of  a  large  number  of  very  narrow  strips  and  the  structure  is 
uniform  only  in  these  narrow  strips — only  such  a  strip  is  to  be  looked 
upon  as  the  elementary  stroma  lamella. 

One  cannot,  by  any  means,  see  the  elementary  lamellae  in  all  places  on  section. 
The  corneal  stroma  is,  unfortunately,  very  subject  to  swelling,  and  wave-like  undula- 
tions or  nickings  of  the  lamellae  are  frequently  present.  So  it  is  clear  that  the  out- 
lines of  the  elementary  lamellae  can  be  seen  only  where  the  section  goes  exactly  at 
right  angles  to  the  surface;  if  the  cut  goes  the  least  bit  obliquely  to  the  border  line,  it 
vanishes  completely.  The  elementary  lamellae  are,  as  a  rule,  better  and  easier  seen 
in  the  posterior  than  in  the  anterior  layers  of  the  stroma. 

When  no  cells  lie  between  them,  the  surfaces  of  the  elementary  lamellae 
are  cjuite  firmly  and  flatly  united  to  one  another;  but  when  cells  are 
found,  the  neighboring  lamellae  are  separated  from  one  another  and  from 
the  cells  in  hardening,  and  in  this  way  the  spaces  already  spoken  of  arise. 

Neighboring  spaces  do  not,  however,  lie  in  the  same  level;  for 
if  one  follows  out  the  whole  surface  expanse  of  a  lamellar-complex 
inclosed  between  two  superimposed  spaces,  one  sees  forthwith  that  it 
divides  up  and  its  component  parts  stick  to  other  lamellar-complexes, 
etc.  In  a  surface  preparation  of  the  cornea  each  cell  requires  a  separate 
focus. 

The  individual  elementary  lamella  consists  of  fine,  straight  connect- 
ive tissue  fibrillae,  strictly  parallel  to  one  another.  However,  the  direc- 
tion of  the  fibrillae  varies  from  lamella  to  lamella.  The  appearance  of  the 
lamellae  upon  cross-section  (PI.  II,  7)  is,  therefore,  diiiferent:  if  the 
fibrillae  run  perpendicular  to  the  cut  surface  {q),  the  lamellae  appear 
portioned  off  in  a  peculiar  way — as  if  the  fibrillae  had  been  divided  into 
small  bundles.  On  the  other  hand,  when  the  fibrillae  course  parallel 
or  nearly  parallel  to  the  cut  surface  (/),  the  lamellae  are  finely  striated  or 
homogeneous.  (High-power  magnification  and  a  narrow  aperture  are 
necessary  to  make  out  this  striation.)  One  recognizes  this  change  of 
direction  as  well  as  the  parallelism  of  the  fibrillae  in  each  individual 
lamella  still  better  in  surface-sections. 

According  to  Tartuferi  (220),  the  corneal  stroma  is  permeated  by 
numerous  elastic  fibers,  coursing  mainly  in  the  direction  of  the  corneal 
lamellae ;  they  lie  between  the  lamellae  and  form  nets  with  beaded  meshes 
(perifascicular  nets);  at  times  there  are  membranous  expansions  at  the 
nodal  points.  These  fibers  arise  from  the  corneal  cells,  according  to 
Lieto  Vollaro  (141)  and  Seefelder  (202).     An  especially  thick  layer  of 


THE  CORNEA  ^3 

such  elastic  libers  lies  just  in  front  of  Descemet's  membrane  and  has 
been  called  the  lamina  elastica  corneae  by  Seefelder. 

According  to  my  observations,  these  elastic  fibers  course  mainly  in 
the  direction  of  those  collagenous  fibrillae  in  the  particular  lamellae  which 
they  support,  as  in  the  sclera.  The  fine  undulations  described  and 
depicted  by  Tartuferi  are  apparently  a  result  of  his  method  of  prepara- 
tion. Seefelder's  drawings  and  my  preparations  show  them  absolutely 
straight  (PI.  II,  8,/). 

The  elastic  fibers  of  the  cornea  are  objects  difficult  of  demonstration 
by  the  microscope;  the  ordinary  staining  methods  do  not  show  them  at 
all,  as  a  rule.  They  can  be  demonstrated  most  easily  by  the  molybdic- 
acid-hematoxylin  method  of  Held  after  fi.xation  in  Zenker's  fluid,  as  See- 
felder has  shown.  This  method  is,  in  general,  a  very  simple  one  and,  too, 
is  indispensable  for  the  demonstration  of  the  corneal  cells. 

The  reason  that  I  have  deferred  discussion  of  the  dimensions  of  the 
elementary  lamellae  is  that  we  have  had  to  rely  very  much  upon  conjec- 
ture in  this  matter.  The  thickness  is  easiest  to  measure:  in  places  such 
as  those  shown  in  PL  II,  7,  where  the  individual  lamellae  have  separated 
from  one  another,  I  estimate  the  thickness  of  the  cross-section  of  the 
lamella  to  be  1.3  mu.  In  other  places  I  have  found  the  thickness  to  be 
as  great  as  2.5  mu.  Pes  (170)  gives  the  thickness  of  an  elementary 
lamella  as  i  mu. 

The  breadth  of  a  lamella,  i.e.,  its  expanse  parallel  to  the  surface 
and  at  right  angles  to  the  direction  of  the  fibrillae,  cannot  be  directly 
measured  with  exactness,  because  one  cannot  follow  the  individual 
lamella  for  so  great  a  distance  with  certainty.  But  it  is  certain  that  the 
figure  of  10  to  20  mu,  which  Pes  gives  as  the  breadth,  falls  far  short  of 
the  actual  breadth. 

The  length  of  the  lamella,  i.e.,  its  expanse  in  the  direction  of  its 
fibrillae,  attains  the  width  of  the  whole  cornea,  according  to  H.  Virchow. 
We  must,  therefore,  look  upon  the  lamellae  as  broad,  thin  bands,  which 
cross  each  other  at  wide  angles,  even  of  90°,  and  lace  in  and  out  among 
each  other  at  very  narrow  angles,  so  that  they  vary  only  slightly  from  a 
course  parallel  to  the  surface. 

In  the  anterior  layers  of  the  cornea  the  spaces  are  shorter  and  the 
variation  from  a  course  parallel  to  the  surface  is  greater  than  in  the 
posterior  layers.  One  must,  therefore,  conceive  that  the  lamellae  in 
front  are  narrower  and  thread  in  and  out  among  each  other  more  than 
do  those  behind. 

This  oblique  fiber  course  is  especially  plain  in  many  places  just  behind 
Bowman's  membrane.     Such  trains  were  called  the  fibrae  arcuatac  by  the 


34  ANATOMY  AND  HISTOLOGY  OF  THE  HUMAN  KVKBALL 

older  authors.  They  ai)poar  to  me  to  have  a  certain  relation  to  the 
nerve  pores  of  Bowman's  membrane,  and  possibly  they  are  only  trains 
of  fibers  which  accompany  the  corneal  nerves,  in  case,  indeed,  they  are  not 
the  nerves  themselves. 

The  greater  thickness  of  the  marginal  parts  of  the  cornea  is  due  to  the 
greater  number  of  lamellae,  found  only  in  the  posterior  layers.  If  one 
follow  these  layers  from  the  periphery  toward  the  center,  for  example, 
one  sees  that  from  place  to  place  the  most  posterior  lamellae  (those 
nearest  Descemet's  membrane)  become  thinner  and  finally  disappear 
(end  in  wedges). 

The  cells  of  the  cornea  are  fixed  and  wandering  cells.  The  fixed 
corneal  cells  lie  in  the  spaces  of  the  stroma  and  for  this  reason  are  flat 
elements  placed  parallel  to  the  surface.  The  cell-body  consists  of  a  deli- 
cate membrane,  which  only  increases  to  a  notable  thickness  in  the 
neighborhood  of  the  nucleus,  as  shown  on  cross-section  (PI.  II,  6,  9). 
On  surface-section  the  cell-bo.dy  is  not  visible  after  ordinary  staining  and 
only  Held's  stain  shows  it  to  be  an  extremely  weak  staining,  finely 
granular  or  reticular  structure  of  irregular  form  with  a  few  broad  pro- 
cesses (PI.  II,  8).  One  gets  a  similar  picture  in  gold  preparations,  as 
drawn  by  Druault  (46)  and  Fuchs  (69).  The  cells  build  a  closed  network, 
or  better  a  trabeculum  01  syncytium,  by  the  help  of  these  processes. 

The  nuclei  of  the  corneal  cells  are  markedly  flattened,  their  thickness 
reaching  only  2  mu  or  less.  Therefore,  in  cross-section  or  when  on  edge, 
they  seem  to  be  very  narrow,  elongated,  and  dense  staining;  on  surface 
view,  on  the  other  hand,  they  stain  very  weakly  and  are  of  a  great  variety 
of  form,  with  rounded  angles,  lobulated,  constricted  like  a  kidney,  or 
elongated.  In  general,  they  are  larger  than  the  nuclei  of  the  sclera; 
rounded  forms  have  a  diameter  of  some  12  mu;  elongated  forms  may 
attain  a  length  of  27  mu.  The  nucleus  has  a  very  delicate  and  fine- 
meshed  chromatin  net  and  i  to  3  very  fine  nucleoli  lying  in  a  district 
poor  in  chromatin. 

According  to  Ballowitz  (14),  each  cell  contains  a  microcentrum  in  the 
neighborhood  of  the  nucleus  in  the  form  of  two  central  bodies  united 
by  a  bridge;  yet  no  radiating  arrangement  of  the  protoplasm  can  be  made 
out  about  this  microcentrum. 

According  to  the  view  of  v.  Recklinghausen  (177),  the  cells  and  their 
processes  lie  in  an  extensively  subdivided  system  of  canals  and  canaliculi 
(lymph  canal  system),  which  provides  for  the  circulation  of  the  fluid  and 
the  nutrition  of  the  cornea.  Since  then  it  has  been  conclusively  shown 
by  Leber  (138)  that  this  view  is  untenable.  The  union  between  the  cells 
and  the  substantia  propria  is  only  more  easily  broken  up  than  is  the  union 


THE  CORNEA  35 

between  the  respective  lamellae  of  the  siibsla)itia  propria,  and  for  this 
reason  one  can  inject  the  canal  system  about  the  cells,  and  for  the  same 
reason  the  wandering  cells  are  principally  found  lying  between  the  fixed 
cells  and  the  substantia  propria. 

The  wandering  cells  are  migratory  leucocytes  with  clear  or  weakly 
granular  protoplasmic  bodies  and  heavy  staining,  lobulated  or  frag- 
mented nuclei  (PI.  II,  8,  w).  Their  form  is  determined  by  the  amount 
of  room  which  the  cell  has;  in  the  clefts  they  are  pressed  down  flat; 
within  the  lamellae  they  appear  to  be  drawn  out  into  long  spindles  con- 
forming to  the  direction  of  the  fibers. 

Blood-vessels  are  entirely  absent  in  the  stroma  of  the  cornea  proper; 
on  the  other  hand,  a  richly  developed  nerve-plexus  is  present;  this  is 
distributed  over  Bowman's  membrane  and  to  the  epithelium,  as  well  as 
to  the  stroma. 

The  Nerves  of  the  Cornea 

Unfortunately  ordinary  stains  do  not  bring  out  the  corneal  nerves  at 
all.  To  demonstrate  them  one  uses  gold  chlorid,  or  better  the  Dogiel 
methyl-blue  method,  which  makes  the  finest  branches  and  the  nerve- 
endings  visible. 

According  to  Hoyer  (107),  60,  and  according  to  Dogiel  (43),  60  to  80, 
small  branches  enter  the  periphery  of  the  cornea;  the  finer  ones  lie  for- 
ward, the  coarser  ones  behind.  They  contain  medullated  as  well  as 
non-medullated  fibers;  the  former  lose  their  medullary  sheaths  0.3  to 
0.5  mm  from  the  margin  of  the  cornea  (Hoyer);  they  give  off  non- 
medullated  fibers  (which  break  up  into  finer  fibrillae,  Dogiel)  at  the  nodes 
of  Ranvier,  as  well  as  at  the  ends  of  the  medullated  fibers. 

In  this  way  an  extensive  branching  of  the  small  trunks  comes  about, 
and  they  and  their  branches  press  toward  the  center  of  the  cornea  and 
toward  the  surface.  The  totality  of  this  branching  and  anastomosing 
forms  the  plexus  proprius  of  the  cornea  (H.  Virchow) ;  the  peripheral  parts 
of  the  cornea  are  supplied  by  the  anterior  branches,  the  middle  by  the 
posterior.  The  plexus  proprius  is  not  found  at  all  in  the  posterior  layers  of 
the  cornea,  is  coarser  and  looser  in  the  middle  layers,  and  becomes  finer 
and  finer  and  more  richly  subdivided  as  it  approaches  the  surface.  It 
ends  near  Bowman's  membrane  in  a  reticular  formation  (the  terminal  net 
of  H.  Virchow). 

From  this  the  perforating  bundles  and  fibers  (the  rami  perforantes 
of  the  older  authors)  are  given  off;  these  go  obliquely  toward  Bowman's 
membrane  and  come  out  through  its  pores.  Thus  they  reach  a  position 
beneath  the  epithelium,  break  up  then  into  fibrillae  and  broaden  out 
between  the  epithelium  and  Bowman's  membrane  as  the  basal  expansion 


36  ANATOMY  AND  HISTOLOGY  OF  THE  HUMAN  EYEBALL 

(H.  \'irchow)  (subepithelial  plexus,  of  the  older  authors).  Part  of  the 
fibrillae  or  branches  from  the  basal  expansion  extend  directly  forward 
in  the  intercellular  spaces  of  the  epithelium  (intraepithelial  expansion). 

The  nerves  in  the  stroma  end  by  little  plates,  according  to  Dogiel; 
these  are  irregular,  cjuadrilateral  or  spade-form,  flat  structures  with 
dentate  or  serrated  edges.  These  endings  are  found  only  along  the 
border  of  the  cornea.  According  to  Dogiel,  the  nerves  do  not  unite  with 
the  corneal  cells. 

Hooked  and  looped  endings  as  well  as  end-skeins  (identical  with  W. 
Krause's  end-bulbs)  are  found  immediately  beneath  the  epithelium  at  the 
limbus  and  also  beneath  the  marginal  portions  of  Bowman's  membrane. 
The  nerves  in  the  epithelium  itself  end  by  round  or  pear-shaped  end-bulbs. 

4.     descemet's  membrane  {Lamina  dastica  posterior) 
(PL  II,  g,  D) 

This  is  a  typical  "glass  membrane." 

By  "glass  membranes,"  in  general,  one  means  highly  refractile,  structureless 
(homogeneous)  membranes  of  great  firmness  and  elasticity;  at  times  thej^  have  an 
indistinct  lamellation. 

Their  homogeneity  is  shown  by  the  fact  that  no  striation  or  reticulation  of  their 
structure  comes  out  even  under  the  highest  magnification  of  their  surface;  moreover, 
it  is  shown  by  the  form  of  broken  and  torn  edges,  for  these  show  an  irregularly  rounded 
or  slightly  angular  line.  Sometimes  the  torn  edge  has  a  terrace  form,  indicating  a 
certain  degree  of  lamellation;  at  times  this  same  formation  comes  out  in  different 
layers  as  a  variation  of  the  staining. 

The  firmness  of  "glass  membranes"  is  shown  by  their  resistance  to  chemical  agents 
and  by  the  lack  of  histolytic  effect  of  pus  in  pathologic  cases.  With  respect  to  their 
elasticity  it  should  be  said  that  they,  the  glass  membranes,  are  not  elastically  dis- 
tensible in  the  sense  that  rubber  is;  they  are  only  elastic  in  so  far  as  they  have  the 
tendency  to  take  on  a  certain  form  of  their  own,  somewhat  as  a  watch-spring  does. 
This  certain  form  is  a  curve  just  the  opposite  of  that  which  the  membrane  has  in  situ, 
i.e.,  the  membrane  shows  a  tendency  to  roll  up  toward  the  side  opposite.  Moreover, 
the  membrane  in  situ  rests  in  a  certain  state  of  tension ;  so  wounds  of  a  glass  membrane 
gape  plainly,  although  not  widely. 

With  respect  to  their  formation,  the  glass  membranes  are  to  be  looked  upon  as 
cuticulae,  i.e.,  as  surface  secretion  products  of  a  layer  of  cells  along  their  base,  i.e., 
along  the  side  (of  the  cells)  turned  toward  the  connective  tissue. 

Descemet's  membrane  can  be  detached  from  the  corneal  stroma  with 
relative  ease,  and  so  shows  two  equally  plain  and  sharp  contours  upon 
cross-section.  In  its  staining  reactions  it  shows  a  distinct  difference  from 
the  corneal  stroma.  Even  with  eosin  a  slight  difference  is  made  out,  and 
this  is  still  plainer  by  Van  Gieson's  stain:  the  stroma  appears  deep  fuchsin- 
red,  the  Descemet's  membrane  rose-red  to  orange-yellow,  according  to  the 


THE  CORNEA  37 

degree  of  differentiation.  Most  striking,  however,  is  the  difference  after 
staining  for  elastic  libers  with  orcein  or  resorcin-acid-fuchsin,  for  then  the 
stroma  is  without  stain,  and  Descemet's  membrane  dense  brown  or  vio- 
let; yet  the  stain  is  by  no  means  as  intense  as  it  is  in  the  case  of  the 
elastic  fibers  or  in  the  intima  of  arteries. 

In  any  case,  there  is  a  great  difference  between  the  Descemet's  and 
Bowman's  membrane,  and  the  name  lamina  elastica  posterior  is,  therefore, 
much  better  adapted  to  Descemet's  membrane  than  is  lamina  elastica 
anterior  to  Bowman's. 

Pathology  furnishes  a  further  difference  between  the  two  membranes.  Not  the 
slightest  trace  of  regeneration  is  found  in  lesions  of  Bowman's  membrane,  while  lesions 
of  Descemet's  membrane  are  covered  over  after  some  time  by  a  new  glass  membrane. 

Descemet's  membrane  is  5-7  mu  thick  in  the  mid-portions;  toward 
the  border  the  thickness  increases  to  8-10  mu.  In  the  immediate 
neighborhood  of  the  border  wart-like  elevations  appear  along  the  inner 
surface,  as  a  rule  (PI.  IV,  i,  w).  These  are  almost  semispherical  or 
semiellipsoidal,  sharply  demarkated  projections  between  which  the  inner 
surface  is  again  smooth.  The  breadth  of  this  wart  zone  (PI.  Ill,  2, 
w-d)  increases  with  age,  as  does  the  thickness  of  the  whole  membrane. 
The  border  proper  of  Descemet's  membrane  and  its  relation  to  the  scleral 
meshwork  will  be  spoken  of  in  connection  with  the  latter. 

5.   THE  ENDOTHELIUM  OF  THE  CORNEA 
(PI.  II,  9,  En) 

This  is  a  layer  of  single  cells  measuring  some  5  mu  in  height  and  18- 
20  mu  in  width.  Unlike  other  endothelia,  these  cells  are  rich  in  proto- 
plasm and  the  nucleus  is  completely  imbedded  in  it.  Upon  cross-section, 
the  cells,  therefore,  appear  almost  rectangular,  with  a  slightly  oval, 
non-prominent  nucleus;  the  protoplasm  is  often  grossly  granular 
or  shows  normal  granular  vacuoles;  the  outlines  of  the  individual  cells 
are  sometimes  oblique  or  curved. 

In  surface  view  the  nuclei  appear  round  (7  mu)  and  are  quite  regularly 
arranged,  so  that  most  of  the  cells  retain  a  rounded  six-sided  form  (PL 
III,  2,  D).  The  cell-outlines  are  not  always  entirely  clear,  because  of  the 
oblique  course  already  spoken  of,  so  that  by  a  change  in  the  focus  a  change 
in  the  form  of  the  cell  is  produced.  But  if  one  compare  these  cells  with 
those  of  other  endothelia,  whose  cell-outlines,  as  is  well  known,  can  only  be 
demonstrated  by  special  methods  of  preparation,  the  cells  of  the  corneal 
endothelium  must  be  said  to  be  sharply  defined. 

From  a  purely  morphologic  standpoint,  therefore,  this  layer  is  more  like  an 
epithelium  than  an  endothelium;    yet  on  the  basis  of  its  developmental  history  one 


38  ANATOMY  AND  HISTOLOGY  OF  THE  HUMAN  EYEBALL 

must  regard  it  as  endothelium  (cf.  chap,  xvi)  and,  furthermore,  it  is  continuous  with 
endothelium  of  the  usual  a])i)earance. 

The  endothelium  extends  over  nearly  the  entire  inner  surface  of 
Descemet's  membrane.  Only  in  the  region  of  the  wart  zone  does  it  take 
on  another  appearance,  especially  noticeable  on  surface  view  (PI.  Ill, 
2) ;  the  cell-outlines  disappear  completely,  the  granulation  of  the  proto- 
plasm likewise,  the  nucleus  becomes  somewhat  larger,  oval,  and  goes  back 
into  the  interspaces  between  the  warts,  so  that  an  irregular  distribution 
of  the  nuclei  results.  The  height  of  the  cells  also  decreases,  especially 
over  the  warts  (PI.  IV,  i),  where  only  a  very  thin  scarcely  distinguishable 
protoplasmic  covering  is  present. 

b)    The  Limbus  corncac 

This,  as  already  indicated,  forms  a  transition  zone  between  the  cornea 
proper  on  the  one  side  and  the  adjacent  tissues  (conjunctiva  and  sclera) 
on  the  other  side.  In  the  limbus  there  are  combined  certain  characteris- 
tics of  the  neighboring  structures,  e.g.,  the  transparency  which  at  least 
a  greater  part  of  the  limbus  has  in  common  with  the  cornea,  the  vasculari- 
zation and  the  richness  in  special  nerve-endings  which  characterize  the 
conjunctiva.  The  limbus,  moreover,  shows  transitions,  for  in  its  territory 
the  stroma  of  the  cornea  goes  over  into  that  of  the  conjunctiva,  of  the 
episcleral  tissue,  and  of  the  sclera. 

The  borders  of  the  limbus  have  already  been  given,  but  since  the 
corneoscleral  border  is  so  difficult  to  recognize  in  stained  specimens,  the 
following  is  given  as  an  aid  to  orientation  for  the  beginner  (PI.  I).  The 
borders  of  Bowman's  and  Descemet's  membranes  can  be  found  even  with 
moderately  low  power;  if  one  think  of  the  ends  of  these  two  membranes 
as  united  by  a  line,  this  line  is  almost  parallel  to  the  outer  part  of  the 
corneoscleral  border  and  lies  0.75  to  i  mm  from  it.  The  difference 
which  the  corneoscleral  border  shows  in  the  horizontal  and  vertical 
meridian  also  comes  out  in  the  line  of  union  between  the  borders  of 
the  two  basal  membranes. 

Accordingly,  one  can  differentiate  only  two  layers  in  the  limbus: 
the  epithelium  and  the  stroma. 

I.      THE   EPITHELIUM   Or   THE   LIMBUS    CORNEAE 

Even  over  the  marginal  portions  of  Bowman's  membrane  the  epi- 
thelium is  somewhat  thicker  than  it  is  in  the  middle ;  over  the  limbus  this 
thickening  increases  and  in  the  neighborhood  of  the  corneoscleral  border 
attains  its  maximum  (80  to  90  mu).  The  back  border  of  the  epithelium 
(toward  the  stroma)  loses  its  straight  course  and  becomes  wavy;  indeed. 


THE  CORNEA  39 

papillary  forms  appear  at  times.  The  thickening  of  the  epithelium  is 
due  to  an  increase  in  the  number  of  layers;  their  number  mounts  up  to 
10  or  more. 

As  in  the  cornea  proper,  the  epithelium  is  stratified  and  squamous; 
only  the  appearance  of  the  basal  cells  is  altered  in  the  region  of  the  limbus. 
While  the  basal  cells  of  the  cornea  proper  are  larger  and  rich  in  proto- 
plasm and  have  nuclei  which  in  size  and  staining  are  no  different  from 
those  of  the  next  layer,  the  basal  cells  at  the  very  limbus  are  at  once 
small,  scant  in  protoplasm,  and  have  small  dense-staining  nuclei.  The 
epithelium  retains  this  kind  of  basal  cells  in  its  further  course  over  the 
conjunctiva  and  they  are  characteristic  for  the  conjunctival  epithelium. 
The  rest  of  the  cells  keep  the  peculiarities  of  the  corneal  epithelial  cells, 
and  the  low  cylindrical  cells  which  appear  on  the  surface  and  are  char- 
acteristic for  the  conjunctiva,  first  appear  beyond  the  corneoscleral  border. 
One  sees,  therefore,  that  the  transition  of  corneal  epithelium  into  con- 
junctival epithelium  does  not  take  place  throughout  the  entire  thickness 
of  the  layer  all  at  once,  but  along  the  base  before  it  does  on  the  surface. 

Since,  too,  the  basal  cells  are  more  closely  pressed  together,  the 
epithelium  of  the  limbus  and  of  the  conjunctiva  shows  a  dark  seam  along 
its  base,  even  by  low  power  (PI.  I).  This  seam  is  often  found  even  at 
the  border  of  Bowman's  membrane,  but  often  first  appears  a  consider- 
able distance  from  it.  It  is  not  rare  to  find  small  islands  of  basal  cells 
strewn  along  and  first  united  into  an  unbroken  line  in  the  conjunctiva 
sclerae. 

2.      THE    STROMA   OF   THE   LIMBUS    CORNEAE 

The  stroma  of  the  outer  layers  loses  its  regularity  even  opposite 
the  marginal  portions  of  Bowman's  membrane.  Beyond  the  limits  of 
this  membrane  it  loses  the  regular  arrangement  of  its  spaces  (parallel  to 
the  surface)  and  takes  on  the  appearance  of  an  ordinary  connective  tissue 
- — ^first  in  the  outer  layers  only,  then  in  the  deeper  ones  as  well.  It  seems 
to  be  especially  the  appearance  of  blood-vessels  which  brings  about  this 
alteration  in  appearance. 

Occasionally  one  sees  fine  capillaries  just  beneath  the  surface  immedi- 
ately outside  the  margin  of  Bowman's  membrane.  Farther  away  the 
lumina  are  more  numerous,  are  encountered  also  in  the  deeper  layers,  and 
are  intermixed  with  larger  vessels  as  well.  This  brings  us  into  the  territory 
of  the  superficial  marginal  vessel-loops,  the  only  vessel-net  of  the  cornea, 
its  sole  organ  of  nutrition.  The  area  occupied  by  the  marginal  vessel- 
loops  shows,  on  meridional  section,  the  form  of  a  triangle  with  its  apex 
at  the  border  of  Bowman's  membrane  and  its  base  continued  into  the 
conjunctiva,  sclera,  and  the  episcleral  tissue. 


40  ANATOMY  AND  HISTOLOGY  OF  THE  HUMAN  EYEBALL 

The  marginal  loop  net  is  supplied  by  the  terminal  branches  of  the 
anterior  ciliary  arteries.  Leber  has  made  a  beautiful  drawing  of  it  (138) ; 
according  to  him,  when  viewed  from  the  front  it  is  seen  to  be  made  up 
of  very  fine,  small  straight  arterial  branches,  which  build  bow-formed 
anastomoses.  From  these  arches  the  final  branches  are  given  off;  these 
are  very  fine  (5  to  6  mu) ;  they  course  on  farther  in  a  meridional  direction 
and  then  suddenly  coil  about  into  the  venules.  The  venous  trunks  of 
the  loops  are,  for  the  most  part,  at  least  twice  as  wide  as  the  arterial 
ones  and  unite  with  their  neighbors  to  form  a  finely  subdivided  net, 
emptying  into  the  episcleral  venous  net. 

On  the  other  hand,  the  layers  lying  deeper,  on  a  level  with  the  sclera, 
are  vesselless  in  the  region  of  the  limbus,  or  at  the  most  provided 
with  a  few  vessel-loops  scarcely  going  beyond  the  border  of  the  sclera. 
These  layers  go  over  into  the  sclera  in  such  a  way  that  at  first  the  lamellae 
preserve  their  transparency,  but  gradually  they  take  on  the  form  and 
staining  reaction  of  scleral  bundles. 

Elastic  fibers  are  also  found  in  the  limbus  but  they  are  very  sparse  in 
comparison  with  those  in  the  adjacent  tissues.'  They  are  most  numerous 
and  extend  farthest  centralward  in  the  superficial  layers  of  the  limbus, 
those  layers  continued  into  the  conjunctiva  and  the  episcleral  tissue; 
they  only  come  out  here  and  there  in  the  deeper  layers.  Their  frequent 
appearance  is  a  sure  sign  of  scleral  tissue. 

I  cannot  refrain  from  once  more  remarking  that  in  many  particulars  the  micro- 
scopic specimen  does  not  appear  to  agree  with  the  findings  in  life.  The  beginner  most 
readily  falls  into  the  error  of  considering  that  the  sclera  reaches  farther  central  than 
is  actually  the  case,  for  he  forms  his  judgment  upon  the  staining  of  the  tissues,  and  the 
darker  staining  of  the  sclera  as  a  matter  of  fact  does  extend  somewhat  beyond  the 
border  into  the  limbus. 

Another  confusion  which  easily  occurs  is  the  following:  the  conjmictiva  and 
the  episcleral  tissue  seem  to  be  sharpened  and  to  end  in  the  limbus,  and  the  sclera 
to  thicken  the  cornea  to  just  this  extent.  In  reality  the  course  of  the  connective-tissue 
bundles,  parallel  to  the  surface,  is  not  essentially  disturbed,  as  one  can  see  best  in  those 
pathologic  cases  in  which  inflammatory  infiltration  of  the  conjunctiva  and  the  episcleral 
tissues  extends  over  onto  the  cornea. 

It  is,  therefore,  entirely  justifiable  to  speak  of  a  conjunctiva  corneae,  i.e.,  the  super- 
ficial layers  of  the  cornea  may  be  considered  as  a  continuation  of  the  conjunctiva  sclerae. 
However,  this  expression  must  be  extended  to  include  not  alone  the  epitheliiun  and 
Bowman's  membrane  but  the  most  superficial  lamellae  of  the  stroma,  as  well. 

The  nerve-endings  found  in  the  limbus  have  already  been  considered 
with  the  nerves  of  the  corneal  stroma. 


'  Although  one  speaks  of  elastic  fibers,  for  short,  only  those  which  are  positively  stained  by  orcein 
or  resorcin-fuchsin  are  meant. 


THE  STRUCTURES  OF  THE  SCLERAL  FURROW  41 

CHAPTER  HL    THE  STRUCTURES  OF  THE  SCLERAL  FURROW 

THE  CANAL  OF  SCHLEMM  AND  THE  MESHWORK  OF  THE  IRIS  ANGLE 

(PL  ni,  I) 

Close  to  the  bottom  of  the  scleral  furrow  (cf.  p.  20),  i.e.,  separated 
from  the  scleral  tissue  by  only  a  thin  layer  of  tissue,  one  or  more  lumina 
are  seen;  these  have  a  closed  endothelial  covering  and  are  usually  con- 
siderably larger  and  more  prominent  than  the  spaces  of  the  meshwork  of 
the  iris  angle  which  lie  farther  inward.  These  lumina  form  the  Schlemm's 
canal,  sijiiis  vciiosits  sclcrae  (Merkel) ,  circulus  venosus  ciliaris  (Leber)  (Sch). 

The  expression  canal  is  incorrect  for  this  structure  in  that  it  relatively 
seldom  is  a  single  elongated  lumen,  and  then  for  a  stretch  of  only  o .  2  to 
0.5  mm  in  a  horizontal  direction;  usually  there  are  two  or  more  lumina; 
they  lie  side  by  side  or  over  one  another  in  the  scleral  furrow.  Seen  from 
the  surface,  Schlemm's  canal  is  like  the  bed  of  a  great  stream  which  flows 
along  undivided  for  a  stretch,  then  is  divided  into  several  branches  for 
a  stretch.  For  this  reason  the  form  of  the  lumen  changes  in  various 
sections  of  the  same  eye. 

The  endothelial  lining  of  Schlemm's  canal  has  the  same  appearance 
as  in  other  vessels  and  forms  an  extremely  thin  membrane  with  nuclei 
projecting  inward.  Aside  from  the  endothelium,  Schlemm's  canal  has 
no  real  wall,  at  least  none  such  as  one  finds  in  other  vessels  of  the  same 
size;  it  seems  to  be  simply  entrenched  in  the  adjoining  tissue.  On  the 
other  hand,  it  is  not  correct  to  say  that  the  endothelium  lies  immediately 
upon  the  sclera,  for  a  loose  tissue,  poor  in  fibers  but  rich  in  cells,  is  inter- 
posed between  the  two,  as  a  rule;  sometimes  this  is  only  a  thin  layer, 
sometimes  it  is  cjuite  well  developed.  One  sees  this  layer  best  in  sections 
stained  with  Van  Gieson;  the  tissue  is  then  sharply  set  off  from  the  deep 
red  of  the  sclera  by  its  yellow  color. 

One  encounters  a  similar  tissue  on  the  chamber  side  of  the  canal, 
usually,  only  there  it  is  less  developed  or  is  not  present  as  a  continuous 
layer  and  is  poorly  differentiated  from  the  neighboring  trabecular  mesh- 
work. On  the  other  side  it  extends  along  the  veins  (i')  going  off  from 
Schlemm's  canal  into  the  sclera. 

Schlemm's  canal  is  in  any  case  closed  off  from  the  spaces  in  the 
meshwork  lying  inward  to  it,  i.e.,  no  visible  breaks  in  its  wall  are 
present.  For  this  reason  only  solutions,  or  the  finest  suspensions,  such 
as  ink,  pass  into  the  canal;  cells  remain  in  the  meshwork  of  the  iris  angle 
and  lie  outside  the  wall  of  Schlemm's  canal. 

On  the  other  side,  however,  the  canal  communicates  freely  with  the 
venous  system  by  means  of  \'essels  given  off  here  and  there  along  the 


42  ANATOMY  AND  HISTOLOGY  OF    THE  HUMAN  EYEBALL 

scleral  side  of  the  canal;  these  go  obliquely  outward  and  backward  into 
the  sclera  and  unite  with  the  anterior  ciliary  veins  (F)  while  still  within 
the  sclera.  An  actual  capillary  net  for  the  supply  of  the  canal  does  not 
exist,  and  it  must  be  that  some  of  the  deeper  limbus  capillaries  give  off 
blood  to  it;  in  any  case  it  seems  to  connect  laterally  with  the  ciliary 
venous  system  as  a  whole,  if  one  may  judge  from  the  direction  of  its  blood 
stream  (Leber,  138). 

In  prepared  specimens  the  lumen  of  the  canal  is  usually  empty  or 
contains  only  a  few  red  blood  cells.  Complete  filling  of  the  whole  canal 
with  blood  only  comes  about  in  a  stasis  of  the  venous  system,  e.g.,  in 
persons  who  are  hanged. 

In  the  question  as  to  whether  or  not  the  canal  contains  blood  or  aqueous 
during  life,  I  am  disposed  to  the  view  that  it  contains  aqueous.  It  is 
certainly  demonstrated  that  its  main  role  is  to  carry  away  the  aqueous, 
and  its  position  to  one  side  of  the  actual  course  of  the  blood-stream 
warrants  the  supposition  that  it  carries  aqueous.  On  the  other  hand,  a 
slight  circulation  disturbance  during  life  or  hypostasis  in  the  cadaver  is 
probably  quite  sufficient  to  fill  it  partially  or  entirely  with  blood  owing  to 
its  open  communication  with  the  veins. 

Possibly  it  is  not  superfluous  to  call  attention  to  the  fact  that  a  hypostasis,  i.e.,  a 
post-mortal  sinking  of  the  blood  owing  to  the  force  of  gravity,  can  play  a  certain  role  in 
the  fairly  well  closed  vessel  system  of  the  eye,  small  as  it  is;  this  is  true  not  only  of 
cadaver-eyes  but  also  of  eyes  enucleated  in  life  which  have  been  placed  in  a  slow- 
working  conservation  fluid.  The  hypostasis  is  shown  by  the  fact  that  the  veins,  or 
even  the  capillaries,  on  one  side  of  the  bulb  are  completely  filled  with  blood,  on  the 
opposite  side,  empty.  The  matter  of  the  side  upon  which  the  filling  is  found  naturally 
depends  upon  the  position  of  the  eyeball  while  the  sinking  is  going  on. 

Schlemm's  canal  occupies  only  about  the  posterior  half  of  the  scleral 
furrow,  and  not  even  all  of  this,  since  the  scleral  roll  (Sw),  springing  axial- 
ward  from  its  back  border,  overhangs  the  furrow  considerably.  The 
meshwork  of  the  iris  angle  (H.  Virchow)  fills  out  the  rest  of  the  depression. 

Considered  in  and  of  itself,  this  peculiar  structure  presents  itself  as  a 
three-sided  prismatic  band,  of  which  the  anterior  edge  is  extremely  sharp 
and  unites  with  Descemet's  membrane  (at  D)  and  the  most  posterior 
lamellae  of  the  cornea  (at  T).  Behind,  it  unites  with  the  scleral  roll,  the 
anterior  surface  of  the  ciliary  body  and,  by  a  devious  way,  with  the  root 
of  the  iris  (i),  as  well.  Its  outer  surface  borders  directly  upon  the  corneal 
and  scleral  tissue  in  front  and  upon  the  inner  wall  of  Schlemm's  canal, 
or  the  loose  tissue  surrounding  it,  farther  backward;  its  inner  surface  is 
free  and  turned  toward  the  chamber  space. 

Its  union  with  neighboring  structures  can  best  be  demonstrated  by 
pure  anatomic  preparations.     If  one  detaches  the  ciliary  body  from  its 


THE  STRUCTURES  OF  THE  SCLERAL  FURROW  43 

insertion  to  the  sclera  from  behind,  one  obtains  quite  a  complete  prepara- 
tion of  the  meshwork.  A  seam,  consisting  of  the  meshwork  and  the 
marginal  portions  of  Descemet's  membrane,  of  whitish  color  and  more 
than  I  mm  in  width,  then  remains  clinging  to  the  anterior  end  of  the 
ciliary  body. 

If  one  now  separate  the  iris  from  the  ciliary  body,  the  innermost 
meshwork,  united  with  the  iris  root  as  well,  stays  attached  to  it,  while 
the  main  mass  of  the  meshwork  along  with  the  border  of  Descemet's 
membrane  remains  hanging  to  the  ciliary  body.  In  this  way  one  sepa- 
rates two  portions,  which,  at  least  in  the  main,  are  different  from  one 
another  in  their  gross  structure  as  well  as  in  their  histologic  composition. 
Seefelder  and  Wolfrum  (205)  have  retained  the  old  name  ligamentum 
pectinatum  for  the  part  remaining  in  union  with  the  iris,  and  support 
Rochon-Duvigneaud  (182)  in  calling  the  rest  the  trabeculum  sclero- 
corneale.  H.  Virchow  (234)  has  recently  introduced  the  term  uveal  for 
the  former  and  scleral  for  the  latter  meshwork. 

These  two  portions  are  of  very  unequal  bulk;  the  uveal  meshwork  is 
a  delicate,  thin  structure  which  one  can  barely  make  out  with  the  dis- 
secting loupe.  By  far  the  greater  part  of  the  whole  structure  belongs 
to  the  scleral  meshwork. 

When  one  studies  a  meridional  section,  the  uveal  meshwork  (j) 
appears  to  be  made  up  of  only  a  few  obliquely  or  longitudinally  cut  sec- 
tions of  delicate  trabeculae  lying  wholly  superficial  along  the  inner  sur- 
face; these  do  not  give  the  idea  of  a  special  formation  at  all  and  are 
entirely  overlooked  by  the  beginner.  One  may  about  as  well  say  that 
only  scleral  meshwork  is  visible  in  meridional  section,  for  it  makes  up 
so  much  of  the  whole  mass. 

The  study  of  surface  and  teased  preparation  as  well  as  of  cut  sections 
is  unconditionally  necessary  for  a  proper  conception  of  the  make-up 
of  the  entire  meshwork.  The  method  of  obtaining  these  was  given  above. 
I  begin  with  the  description  of  the  scleral  meshwork  and  first  its  anterior 
border. 

In  the  first  place  this  border  is  united  with  the  border  of  Descemet's 
membrane.  As  already  noted  (pp.  37-38),  the  marginal  portions  of  this 
membrane  are  characterized  by  warts  and  changes  in  the  endothelium. 
The  actual  border  {D)  is,  apparently,  sharp  and  plain;  the  membrane 
often  maintains  the  same  thickness  to  the  border;  in  other  cases  it  becomes 
rapidly  sharpened  off  near  the  border. 

As  a  matter  of  fact  the  glass  membrane  does  not  stop  here  but  con- 
tinues over  the  trabeculae  of  the  iris  angle  as  a  very  thin  layer.  The 
appearance  of  an  ending  is  only  brought  about  by  the  fact  that  the  glass 


44  ANATOMY  AND  IIIS'IXJLOGV  OF  THE  HUMAN  EYEBALL 

membrane  thins  out  so  suddenly,  and  especially  by  the  fact  that  as  one 
follows  along  Descemet's  membrane  toward  the  root  of  the  iris  one 
necessarily  at  length  comes  upon  a  hole  in  the  membrane  in  the  situation 
where  naturally  the  membrane  stops  in  the  given  section. 

At  the  end  of  Descemet's  membrane  lies  the  anterior  border  ring 
(Schwalbe).  It  sets  immediately  upon  the  outer  surface  of  this  membrane 
or  appears  to  be  imbedded  in  its  substance  (PI.  IV,  i,  vG).  It  is  a 
fiat  bundle  of  circularly  fibrillated  connective  tissue  supported  by  elastic 
fibers,  which  stain  well  with  orcein.  It  varies  in  its  position,  thickness, 
and  breadth  very  much  in  different  eyes  as  well  as  in  different  portions 
of  the  same  eye,  yet  one  seldom  misses  it  completely  in  meridional  sections. 

In  surface  preparations  (PI.  Ill,  2,  vG)  it  stands  out  owing  to  its 
fairly  compact  and  circular  fibrillation,  i.e.,  fibrillation  parallel  to  the 
margin  of  the  cornea;  it  represents  the  most  anteriorly  placed  (corneal) 
area  in  which  one  finds  circularly  fibrillated  connective  tissue  supported 
by  elastic  tissue.  The  most  anterior  spaces  opening  into  the  anterior 
chamber  are  encountered  first  posterior  to  it.  When  the  meridional 
section  goes  through  one  of  these  foremost  spaces  (as  in  PI.  IV,  i,  at  d), 
one  sees  how  the  Descemet's  membrane,  which  is  still  thick  at  the  anterior 
surface  of  the  border  ring,  bends  about  the  margin  of  the  space  and  con- 
tinues over  the  outer  surface  of  the  border  ring  as  a  thin  glass  membrane; 
farther  forward  it  again  merges  into  Descemet's  membrane  or  goes  over 
into  the  deeper  portions  of  the  meshwork. 

The  anterior  border  ring  can  to  a  certain  extent  be  looked  upon  as  the 
superficial  or  circular  root  (with  respect  to  the  anterior  chamber)  of  the 
meshwork.  In  a  histologic  sense  it  is  in  no  respect  different  from  the 
meshes  lying  immediately  posterior  to  it.  Since  its  fibers  leave  their 
meridional  course  and  spread  apart  from  one  another  in  bundles,  the 
thick,  compact  anterior  border  ring  passes  backward  over  into  a  mesh 
of  flat,  compressed,  thin  bands  (PL  III,  2,  right  part  of  the  drawing). 
Glass  membrane  and  endothelium,  somewhat  modified,  continue  over  the 
trabeculae  and  round  out  the  spaces  of  the  meshwork. 

The  most  posterior  lamellae  of  the  cornea  also  go  over  into  the  mesh- 
work, and  this  transition  is  completed  even  o .  i  to  o .  2  mm  in  front  of 
or  axial  to  the  end  of  Descemet's  membrane.  In  meridional  sections  one 
sees  at  this  point  (PL  IV,  i,  T)  a  group  of  longish  nuclei  between  the 
2  or  3  most  posterior  lamellae,  i.e.,  those  immediately  bordering  Desce- 
met's membrane.  TJiese  nuclei  belong  to  the  endothelium.  With  the 
proper  stain  one  also  recognizes  a  lighter  homogeneous  seam  (glass  mem- 
brane) interposed  between  the  endothelial  nuclei  and  the  connective 
tissue;   the  latter  has  a  meridional  fiber-direction,  and  yet  does  not  show 


THE  STRUCTURES  OF  THE  SCLERAL  FURROW  45 

any  elastic  tissue  fibers  by  orcein  staining.  These  first  make  their 
appearance  farther  back,  where  the  division  and  threading  of  the  bundle 
is  richer,  and  the  direction  of  the  fibers  more  nearly  circular.  At  last  the 
trabeculae  take  on  wholly  the  same  appearance  as  those  coming  out  of 
the  anterior  border  ring.  The  border  of  Descemet's  membrane  and  the 
anterior  border  ring  are,  therefore,  to  a  certain  extent  undermined. 

Still  clearer  is  the  transition  of  the  corneal  tissue  into  the  meshwork  in 
teased  preparations  (PI.  Ill,  2,  left  side  of  the  drawing).  The  fiber  mass 
of  the  particular  corneal  lamella  divides  up  into  narrower  bundles,  which, 
however,  are  always  very  flat,  and  bend  about  into  a  meridional  direc- 
tion (r).  These  bundles  contain  a  glass  membrane  and  an  endothelial 
covering  and  become  ordinary  trabeculae  ( Tr')  in  the  course  of  further 
subdivision  and  reticulated  union.  One  may,  therefore,  look  upon  these 
as  constituting  the  deep  or  meridional  root  of  the  meshwork. 

The  trabeculum  sclerocorneale,  or  the  scleral  meshwork,  thereupon 
comes  out  of  both  roots.  Its  trabeculae  are  pressed  down  flat  (per- 
pendicular to  the  surface  of  the  bulb)  and  form  a  mesh  in  which  a  coursing 
of  the  fibers  parallel  and  circular  to  the  surface  predominates.  One  may 
then  just  as  well  speak  of  a  structure  of  fenestrated  lamellae  formed  by 
the  union  of  obliquely  coursing  bundles.  One  obtains  this  impression  in 
meridional  sections  especially.  The  holes  (windows)  in  these  lamellae 
are  drawn  out  in  a  circular  direction  in  the  middle  layers  and  are  of  varying 
size  and  distance  apart,  so  that  the  trabeculae  lying  between  them  are 
sometimes  broader,  sometimes  narrower.  The  spaces  between  super- 
imposed lamellae  do  not  lie  over  one  another. 

The  bulk  of  the  meshwork  (or  the  number  of  the  trabeculae)  increases 
from  before  backward,  partly  because  new  trabeculae  radiate  into  the 
meshwork  from  the  floor  of  the  scleral  furrow,  although  this  only  holds 
true  for  the  portion  lying  in  front  of  Schlemm's  canal,  in  greater  measure 
because  the  lamellae  increase  by  new  branchings.  Whereas  anteriorly, 
behind  the  border  of  Descemet's  membrane,  possibly  3  or  4  lamellae  lie 
over  one  another,  their  number  mounts  up  to  15  to  20  at  the  posterior 
end  (in  front  of  the  scleral  roll). 

On  the  outside  (toward  Schlemm's  canal)  the  trabeculae  lose  their 
dominant  circular  direction,  the  branches  become  more  stellate,  the  meshes 
more  rounded  and  smaller,  the  trabeculae  more  delicate  (Asayama,  9). 

The  individual  trabecula  (PI.  II,  10)  consists  of  four  elements.  Its 
foundation  is  formed  by  a  thick  non-nucleated  bundle  of  collagenous 
fibrillae ;  the  fibrillae  run  parallel  to  the  long  axis  of  the  trabecula,  there- 
fore, mainly  circular  {h).  This  connective  tissue  is  supported  along  its 
surface  by  relatively  thick  elastic  fibers  (/) ;    they  course  in  nearl)-  the 


46  ANATOMY  AND  HISTOLOGY  OF  THE  HUMAN  EYEBALL 

same  direction  as  the  collagenous  fibrillae  and  are,  therefore,  cut  obliquely 
in  meridional  sections  and  appear  as  points,  which  do  not  disappear  upon 
change  of  focus  but,  at  best,  change  their  position.  They  are  not  plainly 
visible  by  ordinary  staining  yet  their  presence  brings  about  a  very  much . 
sharper  defination  of  the  tissue,  a  darker  contour. 

A  glass  membrane  (g)  succeeds  the  elastic  fibers;  this  is  thicker  along 
the  surfaces  than  upon  the  edges  of  the  trabecula.  It  possesses  all  the 
tinctoral  and  morphologic  peculiarities  of  Descemet's  membrane  and  can 
be  followed  into  it.  The  trabeculae  do  not  arise  simply  by  a  splitting  up 
of  Descemet's  membrane,  as  depicted  by  the  older  authors,  but  are  rather 
entirely  covered  over  by  a  continuation  of  Descemet's  membrane. 

The  whole  is  finally  covered  by  an  endothelium  (c)  which,  as  usual, 
shows  no  cell  borders,  so  one  can  recognize  the  individual  cells  only 
through  the  prominent  oval  nuclei.  The  cell-body  usually  forms  an 
extremely  thin  membrane,  at  least  along  the  sides  of  the  individual  trabe- 
culae, which  one  cannot  differentiate  from  the  contour  of  the  membrane; 
the  cells  contain  protoplasm  (are  thicker)  only  in  the  angles  and  corners 
of  the  meshwork  and  partially  fill  out  the  angle.  The  spaces  in  the  mesh- 
work,  which  from  the  course  of  the  fibers  would  otherwise  be  angular, 
are  thereby  rounded  out.  If  the  section  goes  close  to  such  an  angle  it 
encounters  only  the  rounded  endothelial  cell  although  the  converging 
trabeculae  still  appear  separated,  and  one  obtains  the  impression  that 
the  endothelium  forms  a  bridge  between  the  individual  trabeculae  (to 
be  seen  in  most  of  the  places  marked  e). 

The  endothelium  of  the  meshwork  is  also  only  a  continuation  of  the 
endothelium  of  the  cornea.  It  covers  over  all  the  trabeculae  and  so 
clothes  all  the  spaces  of  the  meshwork,  but  is  not  united  in  any  way  with 
the  endothelium  of  Schlemm's  canal.  No  cell-nuclei  other  than  those 
of  the  endothelium  are  found  in  the  scleral  meshwork;  the  entire  struc- 
ture contains  neither  blood-vessels  nor  nerves. 

Two  stains  are  especially  to  be  recommended  for  the  study  of  histologic  structure 
of  the  trabeculae:  the  orcein  stain  and  that  of  Van  Gieson.  The  first  brings  out  the 
elastic  fibers  clearly;  the  glass  membrane  stains  also  slightly  though  far  less  heavily 
than  does  Descemet's  membrane.  Van  Gieson's  stain  colors  all  connective  tissue  an 
intense  red,  the  glass  membrane  rose-red  to  orange-yellow,  the  protoplasmic  body  of 
the  endothelium  a  pale  yellow.  Finally,  absolutely  meridional  sections  are  indis- 
pensable; one  then  gets  pure  cross-sections  of  the  trabeculae  and  clear  pictures,  and 
the  sections  do  not  need  to  be  as  thin  by  half. 

It  should  be  an  easy  thing  in  this  way  to  distinguish  endothelium  from  glass 
membrane — two  layers  which  are  not  differentiated  with  the  necessary  precision  in  all 
contributions  concerning  the  meshwork.  Moreover,  the  endothelium  is  perishable; 
it  is  thrown  oH  in  eyes  with  advanced  cadaverous  appearances,  and  it  likewise  may 
disappear  under  pathologic  conditions. 


THE  STRUCTURES  OF  THE  SCLERAL  FURROW  47 

The  glass  membrane,  however,  is  indestructible,  and  even  in  the  severest  pathologic 
changes  the  cross-section  of  the  trabecula  (aside  from  the  endothelium)  maintains  its 
characteristic  appearance. 

Posteriorly,  the  main  mass  of  the  meshwork  goes  over  into  the  scleral 
roll,  as  stated  above.  This  consists  of  a  large  number  of  wide  and 
narrow  connective-tissue  bundles  with  the  exact  appearance  of  scleral 
fiber-bundles  and  pursuing  an  absolutely  circular  course  (for  dimensions 
see  p.  23).  Peripherally  they  are  supported  by  large  elastic  fibers; 
these  fibers  are  even  a  trifle  larger  than  those  in  the  meshwork.  A  con- 
nective tissue  lies  between  the  circular  bundles  in  the  form  of  a  mattress- 
work  and  completely  fills  out  the  interspaces ;  this  tissue  is  incompletely 
separated  into  bundles  in  which  the  fibrillae  have  a  more  oblique  course. 
The  trabeculae  go  directly  over  into  this  tissue,  and  as  a  rule  one  can  see 
two  rows  of  elastic  fibers  in  every  such  stripe;  these  are  apparently  the 
continuations  of  the  layers  of  elastic  fibers  present  on  the  two  surfaces  of 
a  trabecula.  Anteriorly  and  inward  the  scleral  roll  has  no  sharp  limits; 
the  circular  bundles  become  smaller  and  so  finally  disappear  in  each 
direction. 

Schwalbe  (194)  spoke  of  the  scleral  roll  as  the  posterior  border  ring 
of  the  meshwork  (ligameittum  pcct'uialum  in  his  terminology),  and  this 
name  has,  indeed,  a  certain  justification,  for  the  scleral  roll  varies  con- 
siderably in  its  structure  from  that  of  scleral  tissue.  On  the  other  hand 
the  histologic  peculiarities  are  not  limited  to  the  scleral  roll  but  extend 
to  the  parts  of  the  sclera  lying  outside  and  behind  it  as  well.  Here,  too, 
the  circular  fibrillation  predominates,  the  bundles  are  narrow,  and  the 
large  elastic  fibers  are  present.  Gradually  this  formation  goes  over  into 
ordinary  scleral  tissue  (H.  Virchow). 

The  size  of  the  scleral  roll  shows  individual  variations,  but  it  never 
includes  the  whole  thickness  of  the  meshwork.  A  limited  number  of 
trabecidae  (lamellae)  are  always  excluded,  and  these  course  past  the 
inner  border  of  the  scleral  roll  and  proceed  directly  to  the  fore  surface 
of  the  cUiary  body  and  are  lost  there  in  the  intermuscular  connective 
tissue. 

The  uveal  meshwork  (H.  Virchow,  234),  ligamentum  pectinatum  of 
Seefelder  and  Wolfrum,  is  the  inner  portion,  that  going  to  the  iris  root; 
it  springs  in  part  from  the  inner  surface  of  the  scleral  meshwork  a  slight 
distance  from  the  border  of  Descemet's  membrane,  in  part,  although 
probably  to  a  lesser  extent,  from  the  border  of  the  latter  (PI.  Ill,  2,  i). 

Moreover,  in  its  further  course  the  uveal  meshwork  is  pressed  against 
the  scleral  and,  therefore,  usually  passes  by  and  around  the  sinus  angle  in 
a  bow,  i.e.,  goes  along  the  anterior  surface  of  the  ciliary  body  over  to  the 


48  ANATOMY  AND  HISTOLOGY  OF  THE  HUMAN  EYEBALL 

root  of  the  iris  (PI.  Ill,  i).  As  already  noted,  one  sees  very  little  of 
this  portion  on  a  meridional  section;  only  a  surface  preparation  clears  up 
its  formation  (PI.  Ill,  3).  The  trabeculae  which  make  up  the  uveal 
meshwork  are  not  flattened  down,  as  in  the  scleral  meshwork,  but  rounded 
like  wire,  provided  here  and  there  with  roll-like  thickenings,  so  that  they 
appear  turned  out  as  by  a  lathe;  in  any  case  expansions  come  out  only 
at  the  nodal  points  of  the  meshwork. 

These  trabeculae  form  a  very  loose  reticulum  made  up  of  wide 
I^olygonal  meshes  with  a  tendency  to  stretch  out  in  a  meridional 
direction. 

The  histologic  make-up  of  these  trabeculae  is  the  same  as  that  of 
the  trabeculae  of  the  scleral  meshwork,  except  that  the  elastic  fibers  fail, 
and  the  differentiation  of  the  central  strand  of  connective  tissue  from  the 
glass  membrane  is  less  complete,  its  contours  are  softer  as  the  artist  says. 
Thereby  the  trabeculae  belonging  to  the  uveal  meshwork  are  easily  dis- 
tinguished from  those  of  the  scleral,  even  in  ordinary  staining  (PI.  II, 
10,  /).  The  transition  to  iris  tissue  (PI.  Ill,  3,  /)  is  completed  as 
follows :  the  endothelium  goes  over  into  that  of  the  anterior  surface  of  the 
iris;  the  glass  membrane  vanishes,  the  central  connective-tissue  strand 
becomes  fibrillated  and  so  merges  into  the  connective-tissue  stroma  of  the 
iris.  The  pigmented  cells  of  the  anterior  surface  of  the  iris  (chromato- 
phores)  often  dispose  themselves  along  these  trabeculae,  even  as  far  as  the 
scleral  meshwork. 

In  addition,  the  so-called  iris  processes  appear  here  and  there,  yet 
they  are  not  to  be  found  in  all  eyes.  They  are  cord-like  structures  pro- 
jecting from  the  anterior  surface  of  the  iris  at  the  ciliary  border  and 
consisting  of  the  same  elements  as  iris  tissue;  they  are  considerably 
thicker  than  the  trabeculae  of  the  iris  angle  and  pigmented,  when  the 
iris  itself  is.  These  processes  more  or  less  tortuously  bridge  over  the 
iris  angle  and  unite  with  the  uveal  meshwork.  For  further  details  of 
their  relationship  to  the  relief  of  the  anterior  surface  of  the  iris,  see  chap.  x. 


CHAPTER   IV.     THE   PERICHORIOIDAL   SPACE   AND   THE 
SUPRACHORIOIDEA 

The  perichorioidal  space  is  an  extremely  narrow  cleft  lying  between 
the  inner  surface  of  the  sclera  and  the  outer  surface  of  the  uvea;  it  has 
almost  as  great  an  expanse  as  the  sclera  itself.  Its  forward  limit  is  formed 
by  the  insertion  of  the  ciliary  muscle  into  the  scleral  roll;  behind,  toward 
the  optic  nerve,  the  space  becomes  less  plain  to  the  eye  and  probably 
ceases  altogether  some  distance  in  front  of  the  nerve,  especially  on  the 


THE  PERICHORIOIDAL  SPACE  AND  THE  SUPRACHORIOIDEA       49 

temporal  side  in  the  region  of  the  fovea  centralis.  Extensions  of  this  space 
go  into  the  emissaria  (see  p.  19). 

The  lumen  of  the  perichorioidal  space  is  probably  nil  in  Hfe,  i.e.,  the 
two  border  walls  and  the  lamellae  lying  between  them  adjoin  each 
other  directly.  In  the  hardened  eye,  however,  one  often  finds  this  space 
distended,  especially  when  Mueller's  fluid  is  used  for  fixation. 

The  two  coats  cling  firmly  together  whenever  blood-vessels  go  from 
the  chorioidea  to  the  sclera,  or  vice  versa,  as  in  the  localities  of  the  vortex 
veins  and  the  short  posterior  arteries.  Pathologic  detachments  of  the 
chorioidea,  therefore,  usually  stop  at  the  vortex  veins  or  are  traversed 
by  furrows  at  these  places. 

As  a  section  through  the  entire  eye  shows  best,  the  whole  perichori- 
oidal space  is  traversed  by  delicate  lamellae  going  from  the  uvea  to  the 
sclera,  i.e.,  coursing  from  in  front  and  within,  outward  and  backward, 
but  in  such  an  oblique  direction  that  when  in  situ  they  appear  to  lie 
parallel  to  the  bordering  walls.  These  lamellae  fuse  together  here  and 
there,  and  from  place  to  place  contain  large  round  openings.  The  whole 
perichorioidal  space  is  in  this  way  subdivided  into  smaller  portions, 
which,  however,  communicate  through  the  openings. 

The  number  of  lamellae  lying  over  one  another  in  any  given  place  is 
something  like  6  or  8.  In  the  posterior  part  of  the  perichorioidal  space 
the  lamellae  are  shorter,  the  unions  between  the  uvea  and  sclera,  there- 
fore, more  frequent;  fewer  lamellae  lie  over  one  another.  Forward,  they 
are  longer,  therefore  the  union  is  more  loose  and  the  lamellae  apparently 
more  numerous.  In  the  region  of  the  ciliary  muscle  the  lamellae  gradu- 
ally disappear  between  the  muscle-bundles,  so  that  in  this  zone  the  number 
of  the  lamellae  steadily  decreases  from  behind  forward  and  the  most 
anterior  reaches  of  the  perichorioidal  space  immediately  behind  the 
scleral  roll  seem  entirely  empty. 

Nothing  is  easier  to  make  than  a  surface  preparation  or  a  teased  specimen  of  the 
suprachorioidea ;  one  needs  only  to  tear  ofif  one  of  the  delicate  brown  fragments  which 
always  cling  to  the  outer  surface  of  the  uveal  tract  in  greater  or  lesser  number  with  a 
fine  forceps.  One  may  mount  it  in  glycerin  or  water  and  study  it  without  further 
preparation,  or  stain  as  desired  and  mount  it  in  Canada  balsam.  Only  in  such  a 
preparation  can  one  study  the  arrangement  of  the  suprachorioidal  lamellae;  the 
meridional  section  only  brings  out  the  lamellae  as  extremely  fine  lines,  in  which  nothing 
more  than  nuclei  and  chromatophores  can  be  made  out. 

Each  suprachorioidal  lamella  (PI.  IV,  2)  has  an  endothelial  coat  as  a 
basis;  this  is  an  entirely  transparent,  structureless,  extremely  fine  mem- 
brane with  only  here  and  there  an  oval,  or  somewhat  irregular,  very  flat 
nucleus  and  1-2  fine  nucleoli  {c).  This  membrane  is  supported  by  a  rich 
plexus  of  elastic  fibers  (/").     These  fibers  stain  in  the  usual  way,  notably 


so  ANATOMY  AND  HISTOLOGY  OF  THE  HUMAN  EYKBALL 

hea\'ier  than  those  of  the  sclera,  but  always  much  more  delicately  than 
those  of  ordinary  connective  tissue.  They  are  straight  or  weakly  bowed; 
for  the  most  j)art  they  form  a  plexus,  i.e.,  the  fibers  cross  in  various 
directions  and  the  angular  branchings  and  insertions  here  and  there  seem 
to  form  a  reticulum.  No  particular  direction  predominates;  only  at  the 
margins  of  openings  in  the  lamellae  do  the  fibers  press  together  and  form 
a  sort  of  ring. 

The  chromatophores  {cli)  form  the  third  structural  element  essential 
to  the  suprachorioidea ;  these  are  flat,  branched  cells  whose  bodies  as  well 
as  processes  are  densely  filled  with  fine  brown  pigment;  the  nucleus  is 
oval  or  irregular,  and  likewise  flat.  In  places  where  elastic  fibers  pass  over 
the  cell  the  pigment  fails,  as  a  rule,  and  it  looks  as  if  the  cell  were  cut  in 
pieces,  especially  in  unstained  preparations.  The  form  of  the  cells  varies 
greatly:  in  the  outer  layers  of  the  suprachorioidea  the  cells,  as  in  the 
lamina  fusca  sclerae,  are  plump  and  have  only  a  few  short  broad  pro- 
cesses. In  the  inner  layers  the  processes  are  more  slender,  longer,  and 
plainly  set  off  from  the  nucleated  cell-body. 

The  chromatophores,  which  we  meet  here  for  the  first  time,  are  an  Important  and 
characteristic  constituent  part  of  the  whole  uveal  tract.  Much  as  they  vary  in  their 
form  and  in  their  content  in  pigment  in  the  different  parts,  yet  certain  properties  are 
common  to  all  forms:  Their  pigment  consists  of  very  fine  round  granules,  finer  and  more 
of  a  black  brown  {melanin)  than  the  epithelial  pigment.  In  comparison  with  other 
forms  of  cells  their  processes  are  thick  and  are  pigmented,  as  is  the  cell-body.  The 
number  of  these  processes,  their  length  and  size  vary  within  wide  limits;  there  is  an 
unmistakable  tendency  to  surface  union  in  the  form  of  nets,  or  a  meshwork  in  space. 
Many  times  they  form,  therefore,  a  syncytium.  Yet  one  finds  places  enough  where 
the  chromatophores  are  so  sparse  that  there  can  be  no  thought  of  a  reticulum. 

Muench  (i6o)  believes  he  has  found  a  cross-striation  of  the  protoplasm  of  the 
chromatophores,  especially  in  the  processes.  He  holds  this  cross-striation  to  be 
mainly  the  expression  of  a  very  closely  wound  spiral.  The  muscular  nature  of  the 
chromatophores  follows  from  the  spiral  structure,  according  to  Muench.  In  addition 
to  this  there  is  union  with  nerve-fibers. 

I  will  not  dispute  these  contentions,  especially  since  Lauber  (137)  and  Schock  (191) 
have  corroborated  them,  yet  I  have  not  so  far  been  able  to  see  the  cross-striations.  I 
cannot  see  what  purpose  so  richly  developed  a  net  of  muscular  elements  could  have  in 
the  chorioidea.  According  to  this  view,  the  chorioidea  must  possess  a  much  greater 
contractility  than  the  iris.  But  so  far  as  we  know,  the  chorioidea  has  no  active  motion, 
and,  moreover,  the  experiments  of  Muench  indicate  at  most  only  a  certain  elasticity, 
nothing  more.  Moreover,  the  chorioidea  should  possess  no  motion,  at  least  in  its 
back  portions,  because  then  any  exact  localization  of  visual  impressions  would  be 
impossible. 

All  these  fi.xed  structures  of  the  suprachorioidea  are  so  thin  and  flat 
and  so  united  to  the  endothelium  which  forms  their  groundwork,  that  the 
cross-section  of  an  individual  suprachorioidal  lamella  often  appears  only 


THE  PERICHORIOIDAL  SPACE  AND  THE  SUPRACHORIOIDEA       51 

as  a  simple  fine  contour.  At  best  the  cross-section  has  a  measurable  thick- 
ness only  when  a  nucleus  or  the  body  of  a  pigment  cell  is  involved.  It  is 
hardly  possible  to  say,  therefore,  on  which  side  the  endothelium  and 
on  which  side  the  other  elements  lie.  To  judge  from  the  few  places 
especially  favorable  for  observation,  the  endothelium  lies  sometimes  on 
the  chorioidal,  sometimes  on  the  scleral  surface  side  of  the  lamella,  in 
places,  indeed,  on  both  sides,  probably  as  a  result  of  the  fusion  of 
neighboring  lamellae. 

Moreover,  wandering  cells  are  present  in  varying  numbers.  They 
differentiate  themselves  from  the  endothelium,  with  which  they  may, 
of  course,  be  confounded  by  the  beginner,  by  their  dense-staining  nucleus 
and  the  plain  non-pigmented  but  frecjuently  granular  protoplasmic 
seam. 

The  entire  suprachorioidea  is  without  vessels,  i.e.,  it  possesses  no 
capillary  system  and,  with  the  e.xception  of  the  strands  now  to  be  described, 
no  collagenous  connective  tissue.  Two  large  arteries,  the  a.  ciliares 
posteriores  longae,  do,  indeed,  course  through  the  perichorioidal  space, 
but  they  give  off  no  branches  in  the  suprachorioidea.  The  finer  structure 
of  these  strands  (see  pp.  12-13)  is  as  follows  (PI.  Ill,  4).  The  artery  (.4) 
is  bordered  on  each  side  by  a  strand  of  collagenous  tissue,  which  con- 
tains a  varying  amount  of  smooth  muscle-fibers  (w).  The  latter  are 
connected  with  the  ciliary  muscle,  into  which  the  artery  finally  enters; 
they  have  a  longitudinal  direction  and  are,  therefore,  only  seen  in  cross- 
sections  (PL  III,  4).  Ciliary  nerves  are  found  on  both  sides  of  this  con- 
nective tissue  strand;  one  of  these  is  regularly  larger  (xV,),  the  other 
smaller  {N^).  The  latter  branches  oft"  from  the  larger  while  it  is  still 
within  the  emissarium.  The  larger  nerve  lies  below  the  artery  on  the 
nasal  side,  above  on  the  temporal  side.  The  nerves  are  flattened  down 
at  right  angles  to  the  bulb  wall  and  show  an  oval  cross-section;  the 
whole  strand  is  finally  surrounded  by  suprachorioidal  lamellae. 

The  remaining  isolated  ciliary  nerves  show  the  same  form  and  investi- 
ture. The  fibers  of  the  ciliary  nerve  have  sheaths  of  Schwann  like  all 
peripheral  nerves ;  they  are  nearly  all  medullated  but  in  varying  degrees ; 
the  neurolemma  is  extremely  thin. 

The  ciliary  nerves  give  off  man}'  finer  branches,  which  break  up  into 
still  finer  plexuses  in  the  inner  layers  of  the  suprachorioidea  and  farther 
on  in  the  chorioidal  stroma ;  many  of  these  branches  consist  of  only  a  few 
or  a  single  non-medullated  fiber.  In  the  nodal  points  of  these  plexuses, 
and  here  and  there  in  the  course  of  the  nerve  branches,  larger  ganglion 
cells  are  interposed  (PI.  IV,  2,  n).  These  ganglion  cells  are  all  multi- 
polar, according  to  the  statements  of  the  authorities,  and  serve  vasomotor 


52  ANATOMY  AXI)  HISTOLOGY  OF  THE  HUMAN  EYEBALL 

purposes.     They  ha\e  their  endings  in  llie  bh)()d-vessels  of  the  chorioidea 
(cf.  chap.  v). 

The  presence  of  smooth  muscle-fibers  in  the  suprachorioidea  will  be 
considered  in  the  description  of  its  union  with  the  ciliary  muscle  (chap.  ix). 

CHAPTER  V.     THE  CHORIOIDEA  (CHORIOIDES) 

The  chorioidea  is  a  cjuite  thin,  soft,  brownish  membrane  possessing  a 
certain  degree  of  elasticity,  and  standing  under  a  moderate  tension  during 
life,  for  it  shows  a  slight  tendency  to  gape  on  a  solution  of  its  continuity. 

Its  outer  surface  is  quite  uniformly  brown  and  dull,  on  account  of  the 
suprachorioidal  lamellae  which  cling  to  it ;  its  inner  surface  is  smooth  and 
under  water  shows  a  weak  reflex — the  pigment  epithelium  must  first  be 
teased  away. 

By  transmitted  light,  and,  for  the  most  part,  even  when  against  the 
light  background  of  the  sclera,  one  sees  the  larger  vessels  as  clear  streaks, 
the  interspaces  as  brown  flecks.  The  intensity  of  this  pigmentation 
depends  in  general  upon  the  complexion:  brunettes  have  a  more  densely 
pigmented  chorioidea  than  do  blondes.  The  vessel  system  is  easiest  made 
out  in  the  anterior  portions  of  the  chorioidea,  because  the  chorioidea  is  in 
general  thinner  here,  the  vessels  larger  and  disposed  more  in  a  single  layer. 

That  which  first  strikes  the  observer  are  the  vortices,  figures  formed  by 
the  confluence  of  the  veins.  Each  such  vortex  (PI.  Ill,  5)  includes, 
however,  not  simply  the  veins  of  the  chorioidea  but  also  those  of  the 
two  other  zones  of  the  uvea  (with  few  exceptions) ;  these  veins  from  in 
front  are  quite  straight,  those  from  the  sides  and  behind  are  more  tortuous 
and  the  lateral  ones  form  bows  with  their  convexities  forward.  The  larger 
stems  arising  out  of  the  union  of  these  vessels  lie  in  front  of  the  center 
of  the  vortex,  for  the  most  part,  and,  therefore,  converge  in  their  further 
course  backward;  in  this  way  the  whole  figure  comes  to  have  the  appear- 
ance of  a  sheaf  or  a  fountain.  The  point  where  all  the  vessels  unite  is 
widened  into  an  ampulla  of  i .  5  to  2  mm  (Fuchs,  65),  and  the  somewhat 
narrower  vortex  vein  goes  out  of  this  and  soon  enters  the  sclera  (seep.  18). 

The  position  and  number  of  the  vortices  is  shown  by  the  exit  points 
of  the  vortex  veins,  with  which  we  are  already  familiar  (p.  9).  The 
vortices  lie  about  the  length  of  the  emissaria  in  front  of  and  somewhat 
more  removed  from  the  vertical  meridian  than  do  the  points  of  exit  of 
the  veins.  So  the  vortices  come  to  lie  2.5  to  3  . 5  mm  behind  the  equator 
and,  like  the  veins,  are  grouped  into  two  pairs,  an  upper  and  lower. 
The  distance  between  the  members  of  a  pair  is  about  half  as  great  as  the 
distance  between  the  pairs  (Fuchs,  65).  Not  infrequently  the  number  of 
the  vortices  is  greater  than  that  of  the  veins,  i.e.,  instead  of  one  vortex 


THE  CHORIOIDEA  53 

there  are  two  separated  halves,  and  the  corresponding  veins  unite  within 
the  sclera. 

The  vessel  system  is  more  compact  and  the  interspaces  more  obscured 
by  pigment  in  the  region  of  the  posterior  pole;  to  the  naked  eye  this  part 
of  the  chorioidea,  therefore,  appears  only  flecked  with  brown.  But  with 
the  not  inconsiderable  magnification  of  the  ophthalmoscope  one  can, 
however,  see  the  larger  vessels  here  when  the  interspaces  are  either 
abnormally  heavily  pigmented  (tessellated  fundus)  or  not  pigmented  at 
all  (albinotic  fundus).  In  moderate  pigmentation  of  the  interspaces  the 
fundus  is  uniformly  red. 

From  this  it  follows  that  the  visibility  of  the  chorioidal  vessels  is 
dependent  not  only  upon  the  pigmentation  of  the  spaces  between  the 
vessels  but  also  upon  the  color  of  the  pigment  epithelium.  By  themselves 
the  chorioidal  vessels,  aside  from  the  capillaries,  would  easily  be  seen 
with  the  ophthalmoscope,  as  shown  by  those  pathologic  cases  in  which 
the  pigment  epithelium  and  the  choriocapillaris  are  destroyed  but  the 
other  layers  of  the  chorioidea  are  intact.  The  pigment  epithelium  lies 
over  the  vessel  system  of  the  chorioidea  like  a  brown  veil,  and  the  darker 
this  veil  is  the  more  it  obscures  the  details  of  the  chorioidea. 

In  study  by  the  ophthalmoscope  one  obtains  the  impression  that 
the  larger  vessels  of  the  chorioidea  form  a  richly  divided  network.  This 
is  a  misconception,  however,  as  a  more  accurate  anatomic  study  and  espe- 
cially injection  experiments  (Leber)  show,  and  is  brought  about  by  a 
repeated  crossing  of  the  vessels.  Since  we  see  the  vessels  very  indistinctly 
and  do  not  see  their  thin  walls  at  all,  we  look  upon  the  crossings  as 
anastomoses.  In  reality  these  are  much  more  rarely  present  than  one 
would  think  from  the  ophthalmoscopic  picture;  only  the  capillaries 
form  an  actual  net. 

The  thickness  of  the  chorioidea  is  usually  given  as  very  slight;  but, 
when  one  remembers  that  the  vessels  are  often  empty  after  death  and  the 
whole  membrane,  therefore,  collapsed,  one  must  come  to  the  conviction 
that  the  chorioidea  is  essentially  thicker  than  it  appears  in  most  sections. 
One  must,  therefore,  consider  areas  in  which  the  vessels  are  filled  with 
blood,  then  it  develops  that  the  region  of  the  posterior  pole  has  a  thick- 
ness of  0.22  mm  (183).  Wolfrum  (240)  also  found  a  similar  figure — 
o .  3  mm  for  the  thickest  place.  Toward  the  periphery  the  membrane 
gradually  decreases  to  one-half  (o.  i  to  o.  15  mm). 

In  the  histologic  study  of  the  chorioidea  I  prefer  the  hardening  in  Mueller's  fluid 
to  all  others;  the  hardening  in  formalin,  especially,  is  not  at  all  adapted  to  finer  study, 
because  the  chorioidea  is  always  thereby  much  compressed.  Besides  cut  sections, 
surface  and  teased  preparations  should  be  studied.     To  obtain  these,  one  cuts  a  piece 


54  ANA'l'OMY  AND  HISTOLOGY'  OF    I'lIE  HUMAX  KYKHALL 

of  the  chprioidca  out  of  the  hardened  bulb,  first  brushes  the  ))if^mcnt  e])ithelium 
away  from  the  inner  surface,  and  lays  the  piece,  inner  surface  down,  in  a  dish  filled 
with  water  or  weak  alcohol,  fixes  it  with  a  finger  of  the  left  hand  and  begins  to  tease 
at  the  outer  surface  with  a  fine  forceps.  First  one  removes  suprachorioidal  lamellae 
(and  one  should  study  the  inner  lamellae;  the  nerve-fibers  and  ganglion  cells  are  found 
here)  and  then  one  comes  upon  the  larger  vessels.  One  grasps  the  wall  of  one  and  pulls 
in  the  direction  in  which  the  finer  branches  lie.  In  this  way  one  can  gradually  remove 
the  whole  layer;  finally  there  remains  a  thin  unpigmented  layer:  this  contains  the 
capillaries  and  glass  membrane.  A  separation  of  these  two  in  an  anatomic  way  is 
impossible,  yet  by  tearing  this  membrane  one  very  easily  produces  step-like  borders, 
i.e.,  the  capillary  layer  clings  to  one  fragment,  the  glass  membrane,  a  slight  distance  in 
front,  to  another  in  a  way  entirely  adequate  for  the  study  of  the  finer  structure  of  these 
layers. 

The  cohesion  between  the  vessel  layer  and  the  capillary  layer  is  decidedly  less  than 
that  between  the  other  layers,  otherwise  one  could  not  make  a  pure  preparation  of 
a  great  expanse  of  the  capillary  layer,  but  a  real  space  does  not  exist  between  these 
two.     In  well-stained  specimens  the  connective-tissue  stroma  is  seen  to  be  continuous. 

If  the  suprachorioidea,  which  was  described  in  the  preceding  chapter 
as  a  tissue  filling  out  the  perichorioidal  space,  be  not  considered,  this 
anatomic  preparation  of  the  chorioidea  shows  a  division  into  three  main 
layers:  (i)  The  vessel  layer  {stratum  vasculare);  (2)  The  capillary  layer 
{choriocapiUaris) ;  (3)  The  glass  membrane  {lamina  vitrca,  s.  elastica 
cliorioideae) . 

I.      THE    VESSEL    LAYER 

(PI.  IV,  3,  Gf) 

This  layer  forms  the  main  mass  of  the  chorioidea  and  is  the  bearer  of 
the  macroscopic  markings — those  visible  with  the  ophthalmoscope. 

A  further  division  of  this  layer  into  one  with  vessels  of  larger  caliber 
and  one,  inward,  with  vessels  of  lesser  caliber  (Sattler,  187)  can  be  made 
out  in  the  thicker  parts  of  the  chorioidea.  The  richer  development  of  the 
whole  vessel  system,  and  the  principle  of  the  arrangement  whereby  the 
caliber  of  the  vessels  decreases  from  without  inward,  condition  this  strati- 
fication. 

The  matter  is  not  to  be  taken  thus  literally:  often  enough  one  is  in 
doubt  where  to  place  the  limits  of  the  two  layers,  and  transitions  in 
caliber  are  numerous.  I  cannot,  therefore,  accept  the  view  of  Nuel  (166) 
that  the  separation  of  these  two  layers  is  ecjually  as  sharp  as  that  between 
the  vessel  layers  and  the  choriocapiUaris,  and  I  do  not,  therefore,  recog- 
nize any  "intervascular  space." 

According  to  Nuel,  the  arteries  predominate  in  the  layer  of  larger 
vessels;  this,  also,  is  probably  true  only  for  the  posterior  (polar)  portions 
of  the  chorioidea  and  is  apparently  only  the  result  of  a  sharp  separation 
in  space  which  the  arteries  and  the  venous  trunks  of  the  chorioidal  vessel 


THE  CHORIOIDEA  55 

system  show  (cf.  chap.  xv).  On  the  other  hand  the  veins  very  markedly 
predominate  over  the  arteries  in  the  layer  of  the  middle-sized  vessels. 
This  latter  statement  is  without  doubt  correct,  at  least  in  so  far  as  one 
encounters  more  venous  lumina  than  arterial  lumina  in  this  layer.  Yet 
the  increase  in  venous  lumina  is  certainly  in  part  only  an  apparent  one,  for 
the  veins  are  more  tortuous  and,  therefore,  more  often  cut  across. 

In  the  region  of  the  fovea  centralis,  where  the  chorioidea  attains  its 
maximum  thickness,  the  layer  of  larger  vessels  and  the  perichorioidal 
space  disappears  entirely,  according  to  Nuel,  while  the  smallest  veins 
increase  appreciably  in  size  and  many  times  lie  over  one  another  in  layers. 
In  the  equatorial  parts  of  the  chorioidea  the  separation  of  the  larger 
from  the  smaller  vessels  is  likewise  lost,  for  the  smallest  arteries  and  veins 
go  over  into  the  capillary  layer,  while  the  rest  of  the  vessels  are  broadened 
out  into  a  single  layer. 

Histologically,  the  vessels  show  the  ordinary  structure  (PI.  Ill,  6). 
The  arteries  {A)  have  a  plainly  developed  muscularis,  which  can  be 
followed  into  the  arterioles  (precapillary  branches) ;  there  it  becomes 
reduced  and,  according  to  Wolf  rum  (240),  consists  of  polymorphous 
structures  whose  branches  surround  the  vessel  lumen  like  polyps.  An 
adventitia  of  finely  fibrillated,  almost  homogeneous,  collagenous  tissue, 
traversed  by  thick  elastic  fibers,  follows  upon  the  muscularis. 

The  veins  {V)  have  perivascular  sheaths  {p),  i.e.,  a  second  proto- 
plasmic tube,  provided  with  flat  nuclei,  about  the  endothelial  lumen  (e), 
whereupon  follows  the  connective-tissue  adventitia.  This  is  relatively 
better  developed  about  the  small  vessels  than  about  the  larger  ones,  yet 
varies  greatly  in  its  development  with  the  age  of  the  individual. 

The  chorioidal  stroma  fills  out  the  vessel  interspace.  This  consists 
mainly  of  the  same  elements  as  the  suprachorioidea,  except  that  here 
collagenous  fibrillae  are  added.  From  without  inward  the  stroma  very 
gradually  changes  in  its  make-up ;  the  outermost  layers  still  contain  many 
endothelial  membranes  and  flat  chromatophores,  so  that  they  scarcely 
differentiate  themselves  from  the  adjacent  suprachorioidal  lamellae. 
Indeed,  the  farther  inward  one  goes  the  more  the  dimensions  of"  the 
chromatophores  increase;  their  bodies  become  smaller,  their  processes 
more  slender.  Their  position  changes,  also,  in  that  they  become  related 
to  the  \essel  walls,  i.e.,  their  processes  broaden  out  parallel  to  the  vessel 
wall.  The  endothelium  becomes  less  prominent  or  goes  over  into  con- 
nective-tissue cells,  the  elastic  fibers  become  finer,  collagenous  tissue 
increases  in  amount. 

The  small  vessels  lying  immediately  outside  the  capillary  layer  still 
contain  a  few  chromatophores  in  their  interspaces,  but  inside  this  layer, 


S6  ANATOMY  AND  HISTOLOCxY  01'  THE  PIUMAN  EYEBALL 

i.e.,  inside  the  border  line  between  the  vessel  and  capillary  layer,  no  more 
chromatophores  are  found.  Here  the  stroma  consists  only  of  collagenous 
and  elastic  fibers  mixed  with  a  few  flat  cell-nuclei. 

Numerous  nerve-libers,  the  last  branches  of  the  ganglionated  plexus 
beginning  in  the  suprachorioidea,  traverse  the  chorioidal  stroma,  especially 
in  company  with  the  arteries  («).  Moreover,  one  also  encounters  ganglion 
cells  (g)  in  the  inner  strata  of  the  vessel  layer,  although  only  scatteringly. 

Its  further  branchings  and  endings  have  been  studied  by  Bietti  (24) 
by  Golgi's  method.  According  to  this  author,  a  meshwork  of  the  very 
finest  sort  is  interposed  between  the  above  described  nerve-fibers.  Other 
nets  surround  the  arteries;  their  fibrillae  show  many  varicosities  and  end 
with  club-form  or  spherical  swellings  in  the  vessel  musculature.  Another 
finer  nerve-plexus  lies  beneath  the  lamina  vitrea. 

The  statements  concerning  the  presence  of  smooth  muscle-fibers  in 
the  chorioidal  stroma  (outside  the  vessel  walls)  are  founded  principally 
upon  a  contribution  of  H.  Mueller  (159). 

According  to  this  author,  the  smooth  muscle-fibers  together  with  a 
plexus  of  small  nerve  bundles  lie  along  the  sides  of  the  arteries.  They 
are  easiest  to  find  by  the  side  of  the  long  posterior  arteries  (cf.  p.  51), 
are  present,  as  well,  however,  along  the  short  posterior  arteries.  Accord- 
ing to  Schweigger  (195),  these  smooth  muscle-fibers  lie  by  the  side  of  a 
plexus  of  clear  ganglionated  nerve-fibers  in  the  innermost  vessel  layer. 
Wolfrum  (240),  on  the  other  hand,  absolutely  denies  the  presence  of 
muscular  elements  outside  the  walls  of  the  arteries. 

The  demonstration  of  smooth  muscular  fibers  in  the  chorioidal  stroma 
is  one  of  the  most  difficult  things  in  histology,  and  I  can  say  that  I  have 
not  been  able  to  see  them  there  with  certainty  up  to  this  time,  i.e.,  in  the 
stroma  of  the  chorioidea  proper.  It  is  certain,  on  the  other  hand,  that 
smooth  muscle-fibers  are  present  in  the  suprachorioidea  and,  furthermore, 
far  behind  the  or  a  serrata  (cf.  chap,  ix,  i). 

These  muscle-fibers  play  a  very  subordinate  role,  in  any  case.  The  generalization 
of  Fukala  (70),  therefore,  that  the  whole  uveal  tract,  with  the  exception  of  the  posterior 
pole,  is  invested  with  a  muscle-net,  certainly  is  improbable.  It  can  only  be  explained 
by  a  misunderstanding  and  is  apparently  a  resuscitation  of  the  statement  of  Iwanoff 
(115)  concerning  the  endings  of  the  ciliary  muscle,  and  the  data  of  F.  Eilhard 
Schultze  (196)  concerning  the  reticular  arrangement  of  the  muscle  tissue  along  the 
inner  surface  of  the  ciliary  muscle,  and  the  above  statements  of  H.  Mueller. 

2.     THE  CAPILLARY  LAYER  {ClwriocapUlaris) 
(PL  IV,  3,  C) 
The  layer  can  be  spoken  of  as  characteristic  of  the  chorioidea,  for  the 
most  important  difference  between  the  equatorial  portions  of  the  chori- 
oidea and  the  most  posterior  zone  of  the  orbiciilus  ciliaris  consists  in  the 


THE  CHORIOIDEA  57 

presence  of  this  layer  in  the  one  and  its  absence  in  the  other.  Further- 
more, the  chorioidea  is  different  from  all  the  other  vessel-containing 
portions  of  the  eye  in  that  its  capillaries  are  broadened  out  into  one 
plane,  i.e.,  they  do  not  build  a  meshwork,  as  elsewhere,  but  do  form 
a  net,  which  is,  moreover,  characterized  by  the  width  of  the  individual 
capillary  vessels;  while  elsewhere  the  capillaries  are  so  narrow  that  the 
blood  corpuscles  can  only  pass  one  after  another  in  single  file,  and 
even  then  must  often  stretch  out,  there  is  room  for  several  blood 
corpuscles  side  by  side  in  the  capillaries  of  the  chorioidea.  In  addition, 
local  widenings  of  the  vessel  by  sac-like  distensions  of  the  capillary  wall 
are  frequent. 

The  network  of  the  capillary  layer  is  especially  thick  in  the  posterior 
parts  of  the  chorioidea,  i.e.,  in  the  region  of  the  fovea  centralis  retinae  and 
its  immediate  neighborhood;  its  meshes  are  rounded  and  the  interspaces 
of  the  capillaries  smaller  than  the  lumina  themselves  (PI.  IV,  7).  The 
smallest  arteries  and  veins  pass  from  without  fairly  perpendicular  into 
the  capillary  layer  (Passera,  168),  and  break  up,  star-like,  into  capillaries, 
i.e.,  each  of  these  small  vessels  divides  up  at  once  into  capillaries  upon  its 
entrance  into  the  capillary  layer,  and  these  radiate  out  in  all  directions. 
In  teased  preparations  one  sees  these  small  afferent  and  efferent  vessels  in 
optical  cross-section  only  (.r) ;  one  recognizes  these  relations  better  on 
cross-sections  (PI.  IV,  3,  V). 

Farther  toward  the  periphery  the  meshes  of  the  capillary  net  con- 
tinually become  wider  and  longer  (PI.  IV,  i).  The  main  difference  is 
due,  however,  to  the  fact  that  not  only  do  the  capillaries  show  distentions 
and  great  variation  of  caliber,  but  also  that  the  smallest  arteries  and 
veins  course  in  the  capillary  layer  itself,  and  the  break-up  into  capillaries 
takes  place  by  feathery  or  dendritic  branchings. 

In  the  region  of  the  ora  serrata  the  network  becomes  remarkably 
loose  and  finally  ceases  with  irregularly  projecting  loops  or  exten- 
sions. The  capillary  net  is  more  or  less  interrupted  over  the  recurrent 
arteries  (.4). 

A  net  of  narrow  capillaries  lying  inside  the  choriocapillaris  (cf.  chap,  ix,  4),  is 
found  on  the  border  between  the  chorioidea  and  ciliary  body,  according  to  Sattler 
(187). 

The  capillaries  (PL  III,  7),  as  elsewhere,  consist  of  simple  endothelial 
tubes  strewn  with  oval,  cjuite  densely-staining  nuclei  (e),  which,  according 
to  Wolfrum,  lie  either  in  the  interspace  or  on  the  outer  side  of  the 
capillary  wall,  i.e.,  on  the  vessel-layer  side.  The  interspaces  (interstices) 
of  the  capillary  net  {i)  are  filled  out  by  an  almost  homogeneous  non- 
nucleated  stroma;  special  stains  are  necessary  to  show  its  very  fine 
collagenous  and  elastic  fibrillae. 


58  ANATOMY  AND  HISTOLOCIY  OF  'I'HE  HUMAN  EYEBALL 

The  collagenous  tissue  forms  the  main  mass  of  this  stroma;  it  is  an 
extremely  delicately  fibrillated  tissue,  staining  only  a  faint  rose-red  by 
Van  Gieson.  Outward  it  is  continued  into  the  stroma  of  the  vessel  layer, 
which  in  its  innermost  layers,  as  already  reported,  contains  the  same  con- 
stituents as  the  tissue  filling  out  the  capillary  interstices.  The  elastic 
fibers  are  especially  fine,  and,  according  to  Wolfrum,  are  found  mainly 
in  the  immediate  neighborhood  of  the  capillary  wall;  they  are  united 
inwardly  with  the  elastic  lamella  of  the  glass  membrane  and  outwardly 
with  the  subcapillary  fibrillar  net. 

This  net  (/),  likewise,  consists  of  collagenous  and  somewhat  larger 
elastic  fibrillae  and  separates  the  capillary  layer  from  the  vessel  layer. 
Since  the  places  at  which  the  smallest  arteries  and  veins  go  over  into  the 
capillaries  are  separated  from  one  another  by  relatively  wide  intervals, 
these  expansions  of  the  stroma  lying  between  the  capillaries  and  the 
layer  of  the  smallest  vessels  take  on  a  flattened-out  or  reticular  character, 
and  occasionally  give  the  impression  of  an  independent  layer,  especially  in 
teased  specimens.  Cell-nuclei  also  lie  in  the  subcapillary  fibrillar  net; 
Sattler,  in  his  time,  considered  them  to  be  the  nuclei  of  an  endothelial 
membrane,  but  Wolfrum  now  looks  upon  them  as  nuclei  of  connective- 
tissue  cells,  because  they  belong  to  the  branched  cells  and  have  granular 
protoplasm  (sc). 

One  can  see  the  nuclei  in  question  best  in  surface  preparations  of  the 
capillary  layer  treated  with  a  nuclear  stain,  such  as  haemalaum.  Two 
systems  (or  kinds)  of  cell-nuclei  then  come  forth  in  the  otherwise  unstained 
tissue.  One  system  belongs  to  the  capillaries;  the  nuclei  are  rounded  or 
oval  and  stain  quite  densely;  they  lie  partly  in  the  contour  of  the  capil- 
laries, partly  (apparently)  in  the  lumen.  The  other  system  (the  sub- 
capillary nuclei)  belongs  to  a  plane  lying  farther  outside;  a  slight  turn 
of  the  micrometer  screw  is,  therefore,  necessary  to  focus  upon  the  sub- 
capillary nuclei  if  one  has  previously  had  the  capillary  wall  in  focus.  The 
nuclei  are  larger,  of  more  irregular  form  and  less  densely  stained,  and  do 
not  correspond  in  their  position  to  the  capillaries,  i.e.,  sometimes  they  lie 
over  the  capillaries,  sometimes  over  the  interspaces. 

With  respect  to  these  nuclei,  I  can  only  corroborate  the  view  of 
Wolfrum;  an  endothelial  membrane  such  as  Sattler  postulates  would 
presuppose  at  least  a  cleft  space.  But  no  such  thing  is  to  be  seen  in  well- 
stained  sections;  the  stroma  of  the  capillary  layer  goes  uninterrupted  over 
into  the  stroma  of  the  vessel  layer  and,  indeed,  its  collagenous  as  well  as 
its  elastic  elements. 

A  fine  nerve-plexus  lies  beneath  the  glass  membrane,  therefore  between 
it  and  the  choriocapillaris,  according  to  Bietti  (24)  and  Smirnow  (208). 


THE  CHORIOIDEA  59 

3.     THE  GLASS  MEMBRANE  {Lamina  vilrca  s.  clastica) 

This  possesses  the  physical  and  on  weak  magnification  also  the 
morphologic  peculiarities  of  glass  membranes  in  general.  One  must  use 
a  very  high  magnification  to  recognize,  on  its  outer  surface,  not  a  homo- 
geneous, but  an  extremely  fine  and  faint,  network.  The  torn  edges  of  the 
surface  preparation  (PL  III,  7)  very  often  show  a  terraced  form,  i.e., 
there  are  present  two  contours  which  do  not  coincide,  an  indication  that 
the  glass  membrane  itself  consists  of  two  further  lamellae. 

In  cross-section  (PI.  IV,  3,  Lv)  the  glass  membrane  appears  as  a 
very  highly  refractile  membrane,  some  2  mu  thick,  firmly  grown  to  the 
stroma  of  the  capillary  layer  especially,  whose  elements,  as  already 
stated,  arise  at  least  in  part  from  those  of  the  glass  membrane.  Yet  a 
stroma  is  present  only  in  the  interspaces  of  the  capillary  net;  the  indi- 
vidual endothelial  tubes  lie  immediatel}'  on  the  glass  membrane  without 
any  intervening  layer. 

The  outer  contour  of  the  glass  membrane — that  turned  toward  the 
capillary  layer — is  sharper,  darker,  often  not  exactly-straight,  and  finely 
granular;  the  inner  contour,  turned  toward  the  pigment  epithelium, 
is  more  delicate — "softer"  in  the  sense  of  the  artist — and  entirely  straight 
and  uniform,  except  for  the  depositions  which  appears  in  old  age.  These 
variations  are  also  simply  expressions  of  the  fact  that  the  glass  membrane 
is  made  up  of  two  lamellae. 

Toward  the  entrance  of  the  optic  nerve  the  membrane,  which  in  general 
has  a  quite  uniform  thickness,  becomes  much  thicker  (3  to  4  mu),  and  the 
two  lamellae  can  be  made  out  more  easily.  Concerning  the  ending  of 
the  glass  membrane  at  the  optic-nerve  entrance,  compare  the  anatomy 
of  the  optic-nerve  entrance  (chap,  viii,  a). 

From  their  position  the  two  lamellae  making  up  the  glass  membrane 
can  be  called  the  outer  and  inner;  from  their  nature  they  may  also  be 
distinguished  as  elastic  and  cuticular  lamellae. 

The  outer  or  elastic  lamella  (PL  III,  7,  d)  bears  the  above  reported 
clear  network,  first  described  by  Sattler.  Smirnow  (208)  later  demon- 
strated a  dense  plexus  of  finest  elastic  fibers  in  this  network  by  the  orcein 
stain.  The  Sattler  network  corresponds  to  the  larger  bundles  of  this 
elastic  mesh  only,  and  Smirnow  has  called  it  the  stratum  elasticum  supra- 
capillarc.  As  stated,  this  is  united  with  the  elastic  fibers  of  the  capillary 
interstices  and  to  a  certain  extent  closes  off  the  whole  elastic  system  of 
the  capillary  layer  inward.  In  general,  this  lamella  has  no  measurable 
thickness;  by  itself  it  appears  only  as  a  contour  on  cross-section,  and, 
moreover,  the  elective  elastic  fiber  stain  only  makes  this  contour  sharper, 
not  broader.     In  the  region  of  the  optic-nerve  entrance,  alone,  the  elastic 


6o  ANATOMY  AND  HISTOLOGY  OF  THE  HUMAN  EYEBALL 

lamella  is  ihicker  and  lo  llie  same  extent  that  the  glass  membrane,  in 
general,  is  thickened.  Its  fibrillae  come  out  more  plainly  and  takeOn  a 
more  and  more  circular  course. 

Some  o.i  mm  from  the  edge  of  the  optic-nerve  canal,  where  the 
choriocapillaris  disappears  as  a  closed  layer,  the  subcapillary  elastic  fiber 
net  appears  over  the  glass  membrane  and  this  latter  gives  off  larger  and 
more  numerous  elastic  fibers.  A  circular  mesh  of  elastic  and  collagenous 
fibers  of  considerable  thickness  arises  in  this  way;  on  cross-section  it 
appears  as  a  layer  of  some  2  mu  thickness  and,  on  account  of  the  rich 
mass  of  elastic  fibers  which  it  contains,  it  stains  quite  otherwise  than  the 
neighboring  structures.  As  a  result  of  the  circular  course  of  the  fibrillae, 
this  layer  appears  finely  punctate  or  granular  upon  meridional  section  of 
the  papilla,  but,  on  the  other  hand,  longitudinally  striated  in  sections 
tangential  to  the  border  of  the  papilla. 

The  inner  lamella  (PI.  Ill,  7,  cu)  is  entirely  homogeneous,  like 
other  glass  membranes,  and  is  apparently  a  cuticular  formation  of  the 
pigment  epithelium,  as  shown  by  the  pathologic  cases  in  which  a  certain 
kind  of  regeneration  or  hyperplasia  of  this  lamella  takes  place ;  it  should, 
therefore,  be  called  the  cuticular  lamella. 

By  ordinary  staining  this  lamella  seems  to  form  the  main  mass  of  the 
glass  membrane,  i.e.,  the  cuticular  lamella  makes  up  almost  the  entire 
thickness  of  the  glass  membrane. 

Wolfrum  (240)  recognizes  a  lamina  elaslica  chorioideae  and  a  basal  membrane  of  the 
pigment  epithelium.  The  latter  is  only  about  one-half  as  thick  as  the  former  and  gives 
a  protoplasmic  as  well  as  a  collagenous  tissue-staining  reaction.  A  space,  traversed 
by  finest  collagenous  fibrillae,  lies  between  the  two  membranes.  It  appears  that  these 
details  can  only  be  demonstrated  by  special  staining  methods,  especially  such  as  Held's 
protoplasmic  stain,  and  are,  therefore,  not  \isible  after  ordinary  stains. 

I  have  not  seen  collagenous  tissue  between  the  two  membranes  in  my  preparations 
up  to  this  time,  yet  the  presence  of  this  tissue  has  nothing  improbable  about  it,  since 
such  can  be  easily  demonstrated  in  an  analogous  situation  in  the  ciliary  body  without 
special  staining. 

CHAPTER  VI.     THE  PIGMENT  EPITHELIUM  OF  THE  CHORIOIDEA' 

To  the  naked  eye  this  forms  a  thin,  uniformly  brown  covering  over 
the  inner  surface  of  the  chorioidea.  A  darker,  indistinct  area  about 
twice  the  size  of  the  papilla  (Usher,  230)  comes  out  only  in  the  region  of 
the  fovea  centralis  retinae.  Even  moderate  magnification  shows  a  fine 
flecking,  due  to  the  fact  that  not  all  the  cells  are  so  pigmented  as  to  be 
equally  dark;  this  is  also  visible  by  the  ophthalmoscope:  the  character- 
istic granulation  of  the  fundus  comes  from  the  pigment  epithelium. 

'  For  the  justification  of  the  term  see  p.  13. 


THE  PIG:MEXT  epithelium  of  the  CHORIOIDEA  6i 

The  pigment  epithelium  is  made  up  of  a  single  layer  of  protoplasmic 
cells  with  a  diameter  of  about  i6  mu,  most  of  which  appear  six-sided 
upon  surface  view  (PL  III,  8).  This  hexagonal  form  is  due  to  the 
regular  arrangement  and  uniform  size  of  the  cells;  when  indi\-idual  cells 
are  smaller  than  the  rest,  they  have  less  than  six  corners,  when  larger, 
more.  Viewed  in  this  way,  the  cell-body  appears  uniformly  filled  with 
pigment;  the  round  or  weakly  oval  nucleus,  measuring  about  7  mu  in 
diameter,  is  more  or  less  surrounded  by  pigment;  the  cell  borders,  on  the 
other  hand,  come  out  as  sharp,  unstained  stripes  of  almost  i  mu  width. 
The  so-called  cement  substance  consists  of  neurokeratin,  a  material  which, 
too,  covers  over  the  surface  of  each  cell  on  the  side  toward  the  chorioidea 
in  a  thin  layer  (Kuhnt,  128). 

Seen  in  cross-section  (PI.  IV,  3,  P),  the  cells  appear  more  quadri- 
lateral, their  height  is  only  about  half  their  breadth  (8  mu),  the  pigment 
fills  out  only  the  inner  part  of  the  cell  and  leaves  a  thin  layer  of  the  pro- 
toplasm bordering  the  glass  membrane  outward  entirely  free,  so  that  a 
part  of  the  nucleus  is  very  plainly  visible.  The  outer  half  of  the  proto- 
plasm shows  a  radial  structure,  according  to  Kuhnt.  The  cement  sub- 
stance is  only  partly  seen  on  cross-section — where  it  runs  exactly  at  right 
angles  to  the  direction  of  section.  The  form  of  a  pigment  epithelial  cell 
is  a  low  six-sided  prism,  according  to  this. 

Each  pigment  epithelial  cell  carries  a  large  number  of  fine  processes 
(/>/)  along  its  inner  surface;  these  project  inward  between  the  outer  mem- 
bers of  the  rods  and  cones.  Since  it  is  extremely  difficult  to  obtain  the 
retina  in  situ,  these  pigment  processes  are  very  rarely  seen  in  the  human, 
especially,  since  on  account  of  their  fineness  they  are  only  to  be  made  out  as 
such  in  extremely  thin  sections.  In  sections  of  ordinary  thickness  (15  to 
20  mu)  they  are  fused  into  a  uniformly  pigmented  stripe.  I  can,  there- 
fore, give  nothing  more  in  detail  concerning  their  form;  I  suspect  they 
represent  moulds  of  the  interspaces  between  the  outer  members  of  the 
visual  cells. 

The  relation  of  the  outer  members  of  the  visual  cells  to  the  pigment  epithelium  is 
much  better  seen  in  the  retina  of  amphibians  (frogs,  tritons),  because  these  animals 
have  colossal  rods  and  pigment  epithelium  in  comparison  to  warm-blooded  animals. 
The  phototropic  shifting  of  the  pigment  is  also  much  more  easily  demonstrated  in 
these  animals. 

I  have  measured  the  length  of  the  pigment  processes  in  a  faultlessly 
fixed  human  retina  and  found  it  to  be  5  mu;  it  is,  however,  possible  that 
the  processes  are  still  longer  and  that  the  pigment  only  mounted  up  to 
this  height.  Whether  a  shifting  of  the  pigment  (phototropic  pigment 
displacement)  by  illumination  also  takes  place  in  mammalia,  and  in  inan 


62  ANATOMY  AND  HISTOLOGV  OF  THK  HUMAN  EYEBALL 

in  particular,  has  not  yet  been  definitely  settled  (Garten,  73);   the  dis- 
placement of  the  ])igment  must  in  any  case  be  very  slight. 

The  pigment  granules  (PI.  Ill,  9)  are  rounded  in  the  outer  portion  of 
the  cell,  longish,  like  crystal  needles  in  the  inner  portion,  notably  in  the 
processes. 

With  the  ultramicroscope,  the  rounded  granules  appear  deep  red-brown,  the  long 
ones,  on  the  other  hand,  a  light  brownish  yellow  (Raehlmann,  176).  Kuhnt  (126) 
calls  the  pigment  of  the  epithelium  juscin  on  account  of  its  color  and  form ;  it  is  very 
resistant  to  chemical  influences,  but  bleaches  in  acids  when  in  the  light. 

The  crystal-needle  form  is  not  marked  in  man  and  the  granules  look  more  like 
rods  or  spindles.  According  to  Raehlmann,  a  large  number  of  very  small  particles  in 
the  form  of  light-yellowish  bacterial  rods  are  to  be  seen  only  by  ultramicroscopic 
means,  in  addition  to  the  above-described  pigment  granules.  Moreover,  some  of  the 
fuscin  needles  consist  of  and  are  made  up  of  several  such  short  rods  bound  to  a  longer 
one  by  a  protoplasmic  substance.  This  latter  substance  is  reddish  in  the  darkened 
eye  of  an  animal  and  bleaches  out  in  the  light,  just  as  visual  purple  does,  and  for  this 
reason  Raehlmann  holds  it  to  be  identical  with  the  visual  purple. 

The  pigment  epithelium  of  the  chorioidea  is  especially  distinguished  from  all  other 
pigment  cells  of  the  eye  by  the  elongated  form  of  the  granules.  When,  therefore,  the 
epithelium  of  the  chorioidea  breaks  up  and  its  granules  are  carried  aw^ay,  their  origin 
can  be  recognized  from  their  longish  form.  It  is,  nevertheless,  incorrect  to  reverse  the 
conclusion  that  round  pigment  granules  exclude  origin  from  the  pigment  epithelium  of 
the  chorioidea,  because  first,  it  also  contains  rounded  granules  in  the  normal  state, 
and  second,  its  pigment  may  be  completely  transformed  into  roundish  granules  in 
pathologic  conditions. 

As  one  may  note,  I  have  avoided  the  term  cap  for  the  part  of  the  cell  turned  toward 
the  chorioidea  and  base  for  the  portion  bearing  the  processes.  Much  as  they  are  used, 
they  stand  in  direct  conflict  with  the  fact  that  elsewhere  that  side  of  the  epithelium 
turned  toward  the  mesoderm  is  called  the  base  (cf.  the  expression  "basal  membrane," 
"basal  cells"). 

In  general,  the  pigment  epithelium  of  the  chorioidea  shows  a  very 
uniform  development;  variations  from  the  type  are  found  only  in  the 
region  of  the  fovea  centralis  and  or  a  serrata. 

In  the  region  of  the  fovea  (PL  III,  10)  the  cells  are  higher  (11  to 
14  mu)  and  narrower  (9  to  11  mu),  the  cement  ridges  are  more  delicate 
also;  this  is  the  main  reason  for  the  darker  color  of  this  region.  In 
the  neighborhood  of  the  ora  serrata  one  encounters  exceptionally  large 
cells,  often  with  several  nuclei,  along  with  moderately  large  and  normal- 
sized  cells  (Text  Fig.  Ill,  11);  the  diameter  of  such  cells  may  attain 
60  mu  or  more.  The  regularity  of  the  epithelial  layer  also  suffers  thereby, 
and  the  pigmentation  is  often  quite  lacking  in  uniformity.  At  the  very 
fore  end  of  the  retina  the  cells  are  again  smaller  and  parallel  to  the  border 
of  the  retina;  in  this  way  stripes  arise;  sometimes  these  are  plain,  some- 
times not  (often  somewhat  lighter  than  the  neighborhood) ;  these  separate 


THE  RETINA  63 

the  pigment  epithelium  of  the  chorioidea  from  that  of  the  ciliary  body. 
These  stripes  follow  exactly  the  course  of  the  ora  serrata  or  reproduce  its 
zig-zag  form  in  a  weaker  way.  In  any  case  the  border  zone  of  the  epi- 
thelium is  clearly  seen,  for  the  pigment  epithelium  of  the  ciliary  body 
appears  much  darker  than  that  of  the  chorioidea. 

The  region  of  the  ora  serrata  is  probably  very  often  the  seat  of  light 
pathologic  changes,  especially  in  older  individuals,  which  have  not 
occasioned  any  functional  disturbance  in  life.  These  changes  are  of 
the  same  order  as  those  resulting  from  light  chorioiditis:  partly  a  dis- 
appearance, partly  a  hyperplasia  of  the  pigment  epithelium  and  a  fusion 
of  the  same  to  the  retina. 


CHAPTER  VII.     THE  RETINA' 

The  retina  is  a  soft  inelastic  membrane,  completely  transparent 
during  life  except  for  the  blood  stream  in  the  vessels;  at  the  foramen 
oplicum  chorioideae  it  is  united  with  the  intraocular  end  of  the  optic 
nerve  {papilla  nervi  optici) ;  in  front  it  ends  with  a  more  or  less  dentate 
border  {era  serrata).  At  both  these  places  the  union  with  the  neighbor- 
ing structures  is  a  firm  one;  otherwise  the  outer  surface  of  the  retina  is 
only  united  to  the  pigment  epithelium  in  the  above-described  way,  and 
this  union  is  a  relatively  loose  one;^  a  separation  of  this  connection 
{ablatio  retina,  detachment  of  the  retina)  is,  therefore,  frequent  not 
only  under  pathologic  conditions  but  also  as  an  artefact  and  post- 
mortal appearance.  Artificial  and  post-mortal  detachment  of  the  retina 
stops,  however,  at  the  optic  nerve  and  ora  serrata  (pathologic  detach- 
ment often  extends  beyond  the  ora  serrata).  The  inner  surface  of  the 
retina  is  smooth  and  aside  from  the  slight  bulgings  occasioned  by 
the  larger  blood-vessels  and  from  the  fovea  centralis,  is  even,  as  well, 
and,  therefore,  reflects  light  in  young  individuals  (the  retinal  reflex  of 
ophthalmoscopy) . 

The  retina  is  thickest  at  the  border  of  the  optic  nerve,  especially  above 
and  below  it  (some  0.4  mm),  a  little  less  on  the  nasal  side  and  very  much 
less  on  the  temporal  (PI.  IV,  4).  The  retina  thins  out  toward  the 
periphery  to  about  o.  14  mm,  rapidly  at  first,  then  more  gradually.  Only 
the  temporal  side  forms  an  exception. 


'  In  the  stricter  or  clinical  sense  of  the  word;  cf.  the  remarks  on  p.  13. 

'  According  to  a  newer  conception  (Halben,  Die  Kopulalion  der  Netzhatit  mil  dcr  Adcrliaul  durck 
Konlaktvcrhindung,  Berlin,  1910),  this  union  is  much  more  firm  than  one  has  heretofore  held  it  to  be. 
A  simple  mechanical  pull  is  not  sufficient  to  separate  the  connection  between  the  two  membranes.  A 
primary  insult  to  those  organs  concerned  in  the  copulation,  i.e.,  to  the  rods  or  the  pigment  processes, 
is  necessary. 


64  ANATOMY  AND  IIISI'OLOGY  OF  THH  HUMAN  EYEBALL 

Here  lies  the  fovea  centralis;  this  is  an  obliquely  oval, flattened  funnel- 
form  depression  (PI.  V,  4)  with  its  center  some  3 . 5  mm  away  from  the 
border  of  the  foramen  opticum  chorioideae  and  a  little  below  the  middle 
of  it;  its  size  is  about  the  same  or  a  trifle  greater  than  that  of  the 
foramen  opticum  chorioideae  (Dimmer,  40);  the  horizontal  diameter  may 
measure  up  to  2  mm. 

The  edge  of  the  depression  forms  a  low  wall  {w)  sloping  off  very 
gradually  into  the  normal  level  of  the  retina  but  is  more  steep  toward 
the  center,  and  is  uniformly  bevelled  (clivus,  cl).  The  larger  the  fovea 
the  more  gradual,  the  smaller  the  more  steep  this  declivity  is,  yet  its  angle 
never  attains  more  than  25°.  According  to  Kuhnt  (130),  a  flat  area 
(fundus  foveae)  lies  in  the  center  of  the  fovea,  and  exactly  in  the  middle  of 
this  is  a  small,  very  concave  little  depression,  the  foveola.  According  to 
Dimmer,  however,  a.  fundus  foveae  is  found  only  in  a  very  large  flat  fovea, 
otherwise  the  clivus  goes  directly  over  into  the  foveola,  in  the  region  of 
which  the  radius  of  curvature  of  the  retina  is  only  something  like  o .  04  mm. 

In  the  same  way,  the  relative  thickness  of  the  walls  of  the  retina  in  the 
region  of  the  fovea  is  as  follows:  the  wall  is  somewhat  higher  on  the  nasal 
side  (0.275  to  0.41  mm)  than  on  the  temporal  side  (0.22  to  o. 3 5  mm) ; 
in  the  foveola  the  thickness  of  the  retina  is  0.075  to  o.  12  mm;  all  these 
measurements  are  taken  f  -om  Dimmer. 

The  so-called  retinal  reflexes,  which  are  often  so  marked,  especially  in  young 
hypermetropic  individuals,  have  an  extremely  changeable  appearance  in  the  extra- 
foveal  portions  of  the  retina,  since  the  slightest  alteration  of  the  direction  of  the 
incoming  light  causes  the  reflex  to  disappear,  possibly  to  reappear  elsewhere.  In  the 
foveal  region,  however,  the  retinal  reflexes  have  a  greater  constancy  in  form  and 
presence  because  here  there  are  only  two  places  on  the  inner  surface  of  the  retina 
which  can  so  reflect  light  that  it  will  pass  through  the  aperture  of  the  ophthalmoscope 
and,  therefore,  become  visible  to  the  observer  as  reflected  light  (PL  VII,  i).  These 
places  are  the  wall  and  the  foveola:  the  wall  has  the  form  of  a  ring  and  gives  a  ring- 
form  reflex,  the  so-called  macular  reflex;  the  foveola  acts  like  a  concave  mirror  and 
produces  a  reduced,  inverted  image  of  the  part  of  the  ophthalmoscope  in  front  of 
the  pupil.  The  foveolar  reflex,  therefore,  usually  has  a  crescentic  form,  because  one 
almost  always  uses  a  perforated  mirror  (Dimmer,  39). 

Since  the  clivus  is  directed  so  much  away  from  the  course  of  the  rays  which  enter 
the  eye,  the  light  reflected  from  it  cannot  again  go  out  through  the  pupil;  in  its  sphere 
the  reflex  of  the  inner  surface  of  the  retina  fails  completely,  therefore,  and  this  is  one 
reason  why  the  region  of  the  fovea  appears  darker  than  the  neighborhood  in  the  ophthal- 
moscopic picture;  a  second  one  is  the  slight  thickness  of  the  retina,  a  third  the  denser 
pigmentation  of  the  epithelium  (cf.  p.  61). 

On  anatomic  study  the  district  of  the  fovea  centralis  shows  a  citron 
to  orange-yellow  color  (yellow  spot,  macula  lutea).  The  greatest  inten- 
sity of  this  color  is  found  in  the  immediate  neighborhood  of  the  center 


THE  RETINA  65 

(the  foveola).  The  very  center  itself  has  a  much  more  weak  but  still 
yellow  color  (because  of  the  lesser  thickness  of  the  cerebral  layers  in 
this  situation) ;  the  yellow  color  gradually  fades  away  toward  the  periph- 
ery; it  is  impossible,  therefore,  to  give  accurate  dimensions  for  the  yellow 
spot  although  it  is  usually  larger  than  the  fovea. 

The  lighter  color  (citron-yellow)  occurs  in  young  persons  (Cheval- 
lereau  and  Polack,  31),  the  darker  color  (orange)  in  older  persons. 

The  macula  lutea  is  seen  best  in  not-too-old  cadaver-eyes,  in  which 
the  retina  is  already  clouded.  In  entirely  fresh  eyes,  enucleated  during 
life,  one  sees  the  yellow  color  as  little  as  with  the  ophthalmoscope,  pro- 
vided the  retina  is  transparent  and  in  situ.  The  apparent  reason  for 
this  is  that  the  transparent  yellow  color  (lacquer-color)  lies  upon  a 
brown  background,  or,  to  the  ophthalmoscope,  a  red  background.  In 
the  latter  case  there  is  added  the  factor  that  the  source  of  light  which 
one  ordinarily  uses  for  this  purpose  is  itself  very  yellow.  If  one  uses 
da>'!ight,  on  the  other  hand,  one  can  see  the  yellow  spot  in  darkly 
pigmented  individuals  with  the  ophthalmoscope,  although  under  these 
circumstances  it  seems  considerably  smaller  than  the  macula  in  the 
cadaver  (Dimmer,  42). 

When  one  detaches  the  fresh  retina  immediately  after  the  enucleation 
of  the  living  eye,  broadens  it  out  on  a  slide,  and  studies  it  upon  a  white 
background,  one  at  once  sees  the  yellow  fleck,  as  recently  re-established 
by  Chevallereau  and  Polack  in  a  number  of  cases. 

GuUstrand  (81),  on  the  other  hand,  accounts  for  the  macula  lutea 
as  a  post-mortal  appearance,  because  he  has  succeeded  in  so  detaching 
the  fresh  retina  by  patiently  shaking  it  in  a  physiologic  salt  solution, 
that  it  appears  completely  colorless. 

a)  Microscopic  Anatomy  and  Histology  of  the  Retina 

The  investigation  of  the  structure  of  the  retina  is  one  of  the  most  difficuh  tasks 
in  histology,  not  alone  because  its  extremely  complicated  make-up  can  only  be  analyzed 
by  special  methods,  but  because  the  retina  is  an  extremely  delicate  and  perishable 
structure.  The  retina  is  subject  to  cadaverous  changes  much  earlier  than  other  tissues, 
irrespective  of  whether  the  bearer  of  the  eye  dies  or  the  eye  is  enucleated  during  life, 
and  with  no  other  tissue  of  the  eye  must  one  so  often  and  insistently  ask  himself  the 
question  whether  that  which  one  sees  under  the  microscope  actually  corresponds  to  the 
relations  in  life  or  whether  one  has  before  him  a  post-mortem  change,  or  an  artefact, 
wholly  aside  from  the  matter  of  pathologic  alterations.  The  history'  of  the  investiga- 
tion of  the  retina  is  rich  in  errors  of  this  sort  and  has,  indeed,  even  recently  furnished 
opportunity  for  criticism  (cf.  ^'macula  lutea"). 

From  this  comes  the  demand  that  only  absolutely  fresh,  healthy  material  be  used  in 
the  study  of  the  retina,  which  unfortunately  is  not  so  easy  to  obtain  in  man.  Naturally, 
the  best  are  the  normal  eyes  sacrificed  because  of  large  tumors  of  the  neighborhood. 


66  ANATOMY  AND  HISTOLOGY  OF    THE  HUMAN  EYEBALL 

Severe  injuries  come  second  into  consideration,  eyes  which  must  be  enucleated  immedi- 
ately after  the  injury.  \'et  even  the  freshest  material  must  be  Cjuickly  fixed,  otherwise 
cadaverous  changes  at  once  supervene.  Such  rapidly  working  fixation  fluids  as  osmic 
acid,  35  per  cent  nitric  acid,  concentrated  sublimat  solution,  Zenker's  fluid,  are  best 
adapted  for  this  purpose  and  must  work  from  the  side  of  the  vitreous;  the  eye  must 
come  into  the  fixing  fluid  cut  up — and,  indeed,  it  is  then  a  i)iece  of  good  fortune  to 
obtain  a  wholly  unobjectionable  preparation. 

Every  section  perpendicular  to  the  surface  of  the  retina  shows  very 
plainly,  even  on  weak  magnification,  the  stratification  of  this  membrane. 
This  picture,  surprising  in  its  regularity,  is  brought  about  by  the  fact 
that  two  non-nucleated  layers,  or  layers  which  stain  very  poorly,  alternate 
with  nuclear-rich  ones;  after  nuclear  stains,  such  as  haemalaum,  two 
layers,  especially,  come  out  as  sharply-stained  stripes:  the  two  nuclear 
layers.  A  third  less  intensely  stained  stripe,  inward  to  these,  represents 
the  ganglion-cell  layer.  This  much  is  sufficient  for  a  cursory  orientation 
of  the  layers  of  the  retina. 

On  more  accurate  study  one  recognizes  nine  layers  in  the  retina;  they 
are  given  herewith  from  without  inward:  (i)  layer  of  rods  and  cones; 
(2)  membrana  limitans  externa;  (3)  outer  nuclear  layer;  (4)  outer 
plexiform  layer;  (5)  inner  nuclear  layer;  (6)  inner  plexiform  layer;  (7) 
ganglion-cell  layer;  (8)  nerve-fiber  layer;  (9)  membrana  limitans  interna. 

The  connection  between  the  individual  layers  is  effected  in  part  by 
an  extension  of  the  elements  from  one  to  another,  in  part  by  a  system  of 
special  supporting  fibers.  In  the  description  of  the  individual  layers  one 
begins  with  the  extrafoveal  districts  of  the  retina,  not  because  one  expects 
to  find  the  type  of  the  retina  in  the  structure  of  this  district,  but  because 
this  structure  extends  over  the  greatest  part  of  the  retina  and  any  section 
through  the  eyeball  can  be  made  use  of  for  the  study  of  the  extrafoveal 
district;  the  extreme  peripheral  portions  are  of  course  excluded. 

I.   THE  LAYER  OF  RODS  AND  CONES 

(PI.  IV,  i,SZ) 

On  weak  magnification  this  layer  appears  finely  striated  in  a  direction 

at  right  angles  to  the  inner  surface  of  the  chorioidea;   this  is  due  to  the 

palisade  arrangement  of  its  elements.     The  thickness  of  the  whole  layer 

is  greatest  in  the  middle  of  the  fovea  centralis — in  my  preparations  58  to 

67  mu,  according  to  Greeff  (76)  85  mu;  Dimmer  (41),  on  the  other 
hand,  considers  the  greater  thickness  of  this  layer  in  the  fovea  to  be 
an  artificial  effect.  Toward  the  periphery  the  thickness  decreases  quite 
rapidly,  so  that  only  i  mm  from  the  center  of  the  fovea  the  whole  layer  is, 
indeed,  only  some  40  mu  thick.  Farther  on  the  thickness  decreases  much 
more  slowly  (37  to  40  mu  in  the  equatorial  portions).     Heinrich  Mueller 


THE  RETIXA  67 

(15S)  gave  somewhat  larger  figures  (minimum  40  mu)  for  the  peripheral 
portions.  Even  by  low  power  the  rod-and-cone  layers  can  be  seen  to  have 
two  divisions,  an  outer  less  densely  stained  and  an  inner  more  densely 
stained  one;  the  border  between  the  two  portions  lies  about  half-way 
between  the  ends  of  the  pigment  processes  and  the  mcmbrana  Umitans 
externa.  It  is  the  difference  between  the  outer  and  inner  members  of  the 
elements  which  brings  out  this  division  into  two  portions. 

The  elements  out  of  which  this  layer  is  constructed  are  of  two  kinds: 
The  rods  are  slender  cylindrical  structures  with  a  length  corresponding 
to  the  thickness  of  the  entire  layer,  a  breadth,  however,  of  only  2  mu 
or  less.  Each  rod  is  provided  with  a  somewhat  longer,  slenderer,  highly 
refractile  outer  member  {a),  and  a  shorter,  thicker,  finely  granular 
inner  member  (/). 

The  outer  member  is  double  refracting  and  shows  very  fine  longitudinal  furrows 
(stripes) ;  it  consists  of  a  hull  of  neurokeratin  and  a  contents  made  up  of  small  diagonal 
plates  some  0.5  mu  thick  held  together  in  roulette  form  by  a  cement  substance.  A 
disintegration  of  the  outer  member  into  its  platelets  or  amorphous  droplets  comes  about 
very  easily  after  death  or  in  various  fluids,  and  a  shepherd's  staff  or  looped  bowing 
of  the  ends  apparently  represents  a  fore-stage  of  this  destruction. 

The  inner  member,  of  which  the  protoplasm  is  not  so  highly  refractile,  also  shows 
a  fine  longitudinal  striation  in  the  portions  adjacent  to  the  membrana  liniitans  externa. 
This  is  due  to  the  last  extensions  of  Mueller's  supporting  fibers,  the  so-called  fiber- 
baskets.  The  fiber  apparatus,  a  system  of  fibers  which  course  longitudinally  on  the 
surface  and  within  cross  at  narrow  angles,  lies  in  the  outer  third  of  the  inner  member. 
Furthermore,  the  inner  member  contains  a  diplosome  near  the  outer  end,  according 
to  Held  (93).  From  this  a  thread  {outer  thread)  is  given  off  outward,  going  through  the 
hull  of  the  entire  outer  member.  A  second  thread  extends  inward  (inner  thread), 
and  can  in  any  case  be  followed  to  the  Umitans  externa. 

With  the  exception  of  a  3  to  4  mm  wide  zone  at  the  oni  scrrata,  the 
outer  members  of  the  rods  contain  the  visual  purple,  a  transparent  red 
coloring  matter  which  bleaches  out  rapidly  in  the  cadaver,  but  regener- 
ates in  the  dark,  as  long  as  the  union  with  the  pigment  epithelium  is  pre- 
served. The  visual  purple  makes  the  whole  retina  appear  red,  with  the 
exception  of  the  above-mentioned  peripheral  zone  and  the  rod-free  area 
of  the  fovea.  Yet  one  only  sees  this  in  the  fresh  retinae  of  eyes  which 
have  been  previously  kept  in  the  dark. 

One  cannot  see  the  \dsual  purple  in  man  with  the  ophthalmoscope  because  the 
fundus  (pigment  epithelium  and  chorioidea)  is  already  colored  red;  but  in  animals 
which  have  a  white  fundus,  like  the  crocodile,  a  so-called  tapetuni  retinale,  the  \isual 
purple  is  visible  and  one  can  follow  its  blanching  with  the  ophthalmoscope,  likewise 
in  certain  fishes  (Abelsdorff,  i). 

The  cones  are  flask-form  structures  likewise  possessing  a  thinner  outer 
and  a  thicker  inner  member.     The  outer  member  is  narrowed  conicallv 


68  ANATOMY  AND  HI.STOL()(;V  OF  THE  HUMAN  EYEBALL 

toward  the  apex,  the  inner  member  bays  out,  yet  the  form  and  dimensions 
of  the  cones  vary  a  great  deal  with  their  location. 

The  longest  and  slenderest  cones  are  found  in  the  center  of  fovea  (for 
the  measurements  see  p.  66),  for  their  inner  member  is  only  2 . 5  mu 
thick  (Greeff,  75).  These  foveal  cones  (PI.  V,  3, /Z)  look  more  like 
rods  than  they  do  like  the  other  cones,  but  their  cone  nature  is  made 
clear  by  the  absence  of  visual  purple.  The  extrafoveal  cones  decrease 
in  length  toward  the  periphery,  particularly  in  the  outer  member,  which 
is  reduced  to  a  miniature  cone  of  6  mu  length  at  the  ora  serrata.  The 
inner  member  is  some  3  mu  shorter  than  the  neighboring  inner  members  of 
the  rods  and  takes  on  a  more  and  more  bellied  form  toward  the  periphery 
(up  to  a  diameter  of  7 . 5  mu,  according  to  Greeff). 

With  respect  to  the  finer  structure  there  is  great  similarity  between  rods  and  cones: 
both  show  the  same  constituents.  The  thread  apparatus  is  especially  well  developed 
in  the  cones;  it  consists  of  a  thick  fiber  mesh  and  occupies  two-thirds  of  the  inner 
member. 

The  cones  contain  no  visual  purple. 

The  distribution  of  the  rods  and  the  cones  and  their  relations  to  each 
other  are  best  studied  in  surface  preparations  of  the  retina — in  which 
the  thicker  cones  appear  as  larger  discs,  the  thinner  rods  as  smaller  discs. 

In  the  center  of  the  fovea  is  a  district  containing  only  cones;  it  has  a 
diameter  of  about  0.5  mm — according  to  Fritsch  (63),  only  0.15  mm. 
But  the  very  slender  cones  are  found  only  in  the  middle  of  this  district 
and  in  irregular  arrangement;  toward  the  border  of  this  field  the  cones  are 
notably  thicker  and  are  arranged  in  oblicjue  rows  (Fritsch,  63) ;  a  beauti- 
ful drawing  of  the  cone-mosaic  of  the  fovea  has  been  published  by  Heine 
(92).  Outside  this  field  the  rods  appear;  at  first  they  are  strewn  about 
among  the  cones;  soon,  however,  they  become  united  into  a  simple  circle 
about  each  cone.  Farther  on  the  rods  become  more  and  more  numerous 
and  the  cones  wider  apart  until  some  three  to  four  circles  of  rods  intervene 
between  two  cones  (PI.  V,  2).  This  distribution  is  attained  some  4  to 
5  mm  away  from  the  center  of  the  fovea  and  is  then  maintained  quite 
constantly  to  the  periphery.  In  the  most  extreme  periphery  the  cones 
are  again  relatively  increased. 

2.      MEMBRANA   LIMITANS    EXTERNA 
(PI.  IV,  3,  Le) 

This  is  an  extremely  delicate,  sieve-like,  perforated  membrane, 
visible  as  a  fine,  continuous,  or  streaked  line  only  in  absolutely  perpen- 
dicular sections.  The  holes  correspond  exactly  to  the  elements  of  the 
rod-and-cone  layer  in  number  and  position,  for  they  serve  for  the  exit  of 
the  fibers  going  out  from  these  elements. 


THE  RETINA  69 

The  m.  li  mi  tans  externa  belongs  to  the  supporting  tissue  of  the  retina. 
It  is  connected  with  Mueller's  fibers  on  one  side  and  fine  fibers,  so-called 
fiber-baskets  (see  above),  go  off  from  the  other  side  of  it  (outside). 

According  to  Leboucq  (139),  it,  as  well  as  the  fiber-baskets,  is,  as  a 
whole,  a  remnant  of  the  original  intercellular  cement  of  the  fetal  retinal 
cells. 

The  greater  length  of  the  foveal  cones  makes  the  m.  limitans  externa 
show  a  slight  bulging  inward  in  the  center  of  the  fovea,  the  so-called 
fovea  externa.  What  was  said  concerning  the  greater  thickness  of  the 
rod-and-cone  layer  in  the  fovea  holds  true,  of  course,  for  these  as  well. 

3.      THE    OUTER   NUCLEAR   LAYER 
(PL  IV,  3,  ak') 

This  layer  consists  mainly  of  thickly  placed,  rounded,  or  weakly 
oval  structures  (outer  nuclei)  in  which  a  thin  protoplasmic  mantle  and  a 
densely-staining  nucleus  can  be  made  out.  The  nucleus  forms  so  large 
a  portion  of  the  outer  nuclear  element  that  the  whole  laj'er  seems  to  be 
made  up  solely  of  nuclei. 

The  thickness  of  this  layer  is  about  4 . 6  mu  nasal  to  the  optic-nerve 
entrance  and  is  there  some  8  or  9  nuclei  wide.  From  here  toward  the 
periphery  its  thickness  decreases  gradually  but  only  very  slowly  without 
essential  alteration  of  its  appearance.  Temporal  to  the  optic  nerve  the 
layer  is,  in  general,  somewhat  thinner,  and  in  the  direction  of  the  center 
of  the  fovea  its  thickness  decreases  still  more  to  22  mu  at  the  border  of 
the  rod-free  field,  and  here  it  is  only  4  nuclei  thick.  From  here  on  it 
again  increases  in  thickness,  partly  from  an  increase  in  the  number  of 
nuclei,  but  especially  because  these  are  more  widely  spaced  apart,  and 
attains  a  maximum  of  about  50  mu  in  the  center  of  the  fovea  (PI.  V,  4). 

Even  in  ordinary  preparations  one  can  distinguish  two  kinds  of  nuclei, 
yet  in  my  experience  this  difference  is  but  little  noticeable  in  preparations 
from  Mueller's  fluid  and  much  more  striking  in  formalin  and  sublimat 
fixation.  This  difference  affects  only  the  nuclei.  The  one  kind  of  nuclei 
is  smaller  (5 . 7  mu),  more  rounded,  and  more  densely  stained  (PI.  IV,  3,  s) ; 
the  other  is  larger,  plainly  oval  (5X7  mu)  and  more  weakly  stained  (PI. 
IV,  3,  s). 

The  main  bulk  of  the  outer  nuclear  layer  is  made  up  of  the  first  form 
while  in  the  extrafoveal  portions  of  the  retina  those  of  the  second  form 
are  found  only  outside,  right  next  to  the  m.  limitans  externa.  In  the 
territory  of  the  rod-free  field  of  the  fovea  only  nuclei  of  the  second  form 
are  found  and  the  entire  thickness  of  the  nuclear  layer  is,  therefore,  made 
up  by  them.  About  this  field  the  nuclei  of  the  first  form  appear,  at  first 
in  the  innermost  layers  of  the  nuclear  layer,  then  they  rapidly  increase  in 


70  ANATOMY  AND  HISTOLOGY  OF  THE  HUMAN  EYEBALL 

number,  while  the  more  sparse  appearing  nuclei  of  the  second  form  make 
an  unbroken  layer  immediately  along  the  m.  limitans  externa  only.  Farther 
away  from  the  fovea  the  number  of  these  nuclei  continues  to  decrease,  they 
are  more  and  more  separated  from  one  another,  and  the  intervals  are  filled 
with  the  nuclei  of  the  first  form,  which  henceforth  make  up  the  main  mass. 

From  this  distribution  alone,  it  may  be  suspected  that  the  smaller, 
denser  stained  nuclei  of  the  first  form  belong  to  the  rods,  the  larger,  more 
weakly-stained  nuclei  of  the  second  form  to  the  cones.  As  a  matter  of 
fact  each  element  of  the  rod-and-cone  layer  is  bound  to  an  outer  nucleus 
by  a  fiber,  which  one  can  see  without  the  employment  of  unusual  meth- 
ods of  preparation,  for  example,  in  sections  through  the  middle  of  the 
fovea,  where  the  outer  nuclear  layer  is  especially  loose  (PL  V,  3). 

Each  rod,  for  example,  is  extended  inward  as  a  fine,  tortuous,  and 
varicosed  fiber  (rod-fiber) ;  this  goes  through  the  corresponding  hole  in 
the  w.  limitans  externa  into  the  outer  nuclear  layer  and  at  a  varying 
distance  from  this  membrane  distends  into  the  rod  granule  containing 
one  of  the  smaller,  denser  stained  nuclei;  the  protoplasmic  mantle  of 
the  granule  is  extremely  thin,  so  that  it  seems  to  consist  almost  solely 
of  the  nucleus.  The  fiber  continues  beyond  the  cell  and  ends  in  a  small 
bud  in  the  outer  plexiform  layer. 

In  the  same  way  each  cone  goes  over  into  a  fiber  (cone-fiber),  but 
this  is  much  thicker  in  its  distal  part,  i.e.,  between  the  cone  and  its  granule, 
than  is  the  rod-fiber,  and  very  short,  because  the  cone  granule  is  placed 
just  within  the  m.  limitans  externa.  The  cone  granule  is  somewhat  more 
spindle-form,  because  its  nucleus  is  oval  and  somewhat  more  protoplasm 
is  present  at  the  two  ends  of  the  granule.  The  proximal  part  of  the 
fiber  is  throughout  longer  than  the  distal  and  more  slender  than  it,  but 
still  is  always  heavier  than  a  rod-fiber.  The  cone-fiber,  likewise,  ends  in 
the  outer  plexiform  layer,  but  at  a  deeper  level  than  the  rod-fiber,  and, 
indeed,  with  a  conical  swelling  (cone  swelling,  or  cone-foot)  from  which 
short  lateral  branches  go  off. 

Only  in  the  center  of  the  fovea,  where  cones  only  are  present  and  the 
cone  granules  must  be  placed  over  one  another  in  layers,  is  the  distal 
portion  of  the  cone-fiber  thinner  and  longer. 

The  direction  of  the  rod-and-cone-fiber  in  the  extrafoveal  parts  of  the 
retina  is  perpendicular  to  the  surface.  In  the  region  of  the  fovea  the  direc- 
tion becomes  an  oblique  one  and  in  keeping  with  this  the  outer  nuclei 
are  arranged  in  oblique  rows. 

Concerning  the  presence  of  cross-striations  in  the  outer  nuclei,  i.e.,  an  arrangement 
of  the  chromatin  substance  in  cross-bands,  opinions  are  much  divided.  I  can  discover 
nothing  of  this  sort  in  my  preparations. 


THE  RETINA  71 

Here  and  there  the  cone  nuclei  lie  beyond  the  m.  limilans  externa  (the  nucleus  then 
seems  to  lie  in  the  inner  member  of  the  cone)  and  the  affected  cone  shows  an  abnormal 
structure.     The  significance  of  this  appearance  is  not  yet  clear. 

4.     THE  OUTER  PLEXIFORM  LAYER  {Intcmiidcar  layer) 
(PI.  IV,  3,  ap) 

This  layer  shows  a  thickness  of  some  20  mu  in  the  extrafoveal  por- 
tions of  the  retina,  contains  no  cell-nuclei,  and,  therefore,  takes  only  tissue 
stains,  such  as  eosin.  It  consists  of  a  densely  interwoven  reticulum  of 
fibrous  elements  arranged  principally  in  two  directions — perpendicular  to 
the  surface  of  the  retina  and  parallel  to  it  (at  least  these  directions  pre- 
dominate over  the  oblique  ones).  This  structure  is  difficult  to  analyze 
and  by  low  power  and  defective  staining  gives  the  impression  of  granu- 
lation; therefore,  this  layer  was  called  the  outer  granular  layer  by  the 
older  authors;  by  higher  magnification  and  better  staining  of  thin 
sections  it  has  a  fine  reticular  appearance. 

Two  well-separated  portions  can  be  made  out,  an  outer  and  an  inner. 
The  outer  portion  (/)  is  the  thicker;  it  includes  some  two-thirds  of  the 
entire  layer;  it  is,  however,  very  subject  to  swelling  and  often  appears  still 
thicker.  The  meshes  are  loose  and  a  direction  parallel  to  the  surface  rules. 
It  is  the  proximal  portions  of  the  rod-cone  fibers  (see  above)  which  call 
forth  this  picture,  and  the  border  between  the  two  portions  is  formed 
by  the  proximal  ends  of  the  cone-fibers  (the  previously  reported  cone 
swellings),  which  all  lie  at  the  same  level. 

The  inner  portion  (r)  is  much  thinner  and  shows  the  fine  reticular 
(plexiform)  structure  in  a  typical  way;  it  is,  therefore,  more  closely 
meshed  and  stains  more  densely  than  the  outer  portion.  It  is  made  up 
mainly  of  the  fine  extensions  of  the  horizontal  cells  of  the  inner  nuclear 
layer  (see  the  same).  In  addition,  Mueller's  supporting  fibers  have  a 
part  in  the  formation  of  the  entire  layer;  their  fine  extensions  run  parallel 
to  the  surface. 

In  the  neighborhood  of  the  fovea  the  appearance  of  the  outer  plexi- 
form layer  changes  in  a  remarkable  way.  The  rod-and-cone-fibers  take 
on  a  more  and  more  oblique  direction  and  finally,  in  the  immediate 
vicinity  of  the  foveal  center,  are  arranged  almost  parallel  to  the  surface. 
Thereby,  as  well  as  through  the  marked  condensation  of  the  cone-fibers, 
which  are  exclusively  present  in  the  center  of  the  fovea,  the  outer  portion 
loses  every  trace  of  a  reticular  meshwork  and  takes  on  a  fibrous  appear- 
ance. This  modification  of  the  outer  plexiform  layer  bears  the  name  of 
Henle's  outer  fiber  layer  (PI.  V,  3,  4,  Hf). 

This  layer  can  be  made  out  even  at  the  temporal  border  of  the  papilla ; 


72  ANATOMY  AND  HISTOLOGY  OF  THE  HUMAN  EYEBALL 

the  more  one  approaches  the  center  of  the  fovea,  the  longer  the  cone- 
fibers  become,  the  more  they  are  superimposed  in  layers,  and  the  thicker 
the  whole  fiber  layer  is.  It  attains  a  maximum  thickness  of  40  to  50  mu 
at  about  the  border  of  the  rod-free  field,  then  thins  rapidly,  and  is  reduced 
to  a  minimum  in  the  very  center  of  the  fovea. 

Since  the  outer  fiber  layer  is  equally  well  developed  on  all  sides  of 
the  fovea,  a  nearly  circular  area  of  almost  8  mm  diameter  concentric 
with  the  fovea  is  formed  in  which  the  fibrillation  has  a  component 
directed  radial  to  the  foveal  center. 

The  inner  portion  of  the  outer  ple.xiform  layer  maintains  the  same 
appearance  and  the  same  thickness  in  the  region  of  the  fovea  as  in  extra- 
foveal  parts  of  the  retina. 

It  is  very  difficult  to  obtain  good  preparations  of  the  outer  fiber  layer,  for  this 
layer  is  to  a  particular  degree  subject  to  swelling.  This  is  also  the  reason  why  the 
region  of  the  fovea  is  so  easily  detached  post-mortem. 

5.       THE   INNER   NUCLEAR   LAYER 
(PI.  IV,  3,  /A-) 

This  layer  has  a  thickness  of  about  30  mu  in  the  extrafoveal  portions 
of  the  retina ;  it  is,  therefore,  appreciably  thinner  than  the  outer  nuclear 
layer,  to  which  it  is  very  similar  in  appearance  in  ordinary  sections.  The 
inner  nuclei,  which  make  up  this  layer,  are  closely  placed  together;  indeed, 
the  layer  consists  almost  wholly  of  cell-nuclei  with  thin  mantles  of  proto- 
plasm from  which  processes  of  varying  number  and  direction  go  off. 
Here  and  there  one  finds  larger  cells  with  a  rich  protoplasm,  wholly  of  the 
appearance  of  ganglion  cells,  and  provided  as  well  with  Nissl's  granules; 
part  of  the  nuclei  also  show  a  more  elongated  form ;  these  are  the  nuclei 
of  Mueller's  fibers  and  lie  about  the  middle  of  the  inner  nuclear  layer. 

This  is  nearly  all  one  can  see  by  the  usual  staining  of  cut  sections. 
The  extremely  complicated  structure  of  this  layer  is  only  revealed  by 
the  methods  of  Golgi  and  Ramon  y  Cajal. 

According  to  Greeff  (75),  one  can  distmguish  the  following  elements  in  the  inner 
nuclear  layer: 

(i)  The  horizontal  cells,  and,  indeed, 

a)  the  outer  horizontal  cells:  these  are  small  flat  cells  whose  processes  broaden 
out  in  a  direction  parallel  to  the  surface  and  end  in  the  outer  plexiform  layer.  They  lie 
in  the  outer  portion  of  the  inner  nuclear  layer. 

b)  The  inner  horizontal  cells:  these  are  larger  than  the  former  and  likewise  broaden 
out  in  a  direction  parallel  to  the  surface;  their  end  branches  mount  up  toward  the 
outer  plexiform  layer.  Some  of  these  have  a  descending  (pro.ximal  or  inward  directed) 
process  as  well,  ending  in  the  inner  plexiform  layer.  They  lie  at  a  plane  farther  inward 
than  do  the  outer  horizontal  cells. 

2.  The  bipolar  cells.     According  to  their  union,  these  are  di\aded  into: 


THE  RETINA  73 

a)  The  rod  bipolars:  each  of  these  has  a  basket  of  ascending  (distal  or  outward) 
processes  and  by  means  of  these  is  in  contact  with  the  ends  of  the  rod-fibers.  The 
descending  (proximal)  process  is  a  single  fiber  coursing  through  the  inner  plexiform 
layer,  and  invests  a  cell  of  the  ganglion-cell  layer  by  means  of  its  less  thick  branches. 

b)  The  cone  bipolars :  these  lie  ver>'  close  to  the  outer  plexiform  layer,  their  ascend- 
ing (distal)  processes  broaden  out  parallel  to  the  surface  of  the  retina  and  come  in  con- 
tact with  the  prox-imal  ends  of  the  cone-fibers.  The  descending  (proximal)  process  ends 
in  the  inner  plexiform  layer  with  branches  parallel  to  the  surface.  Some  of  these  cells 
are  characterized  by  especially  numerous  ascending  processes  (giant  bipolars,  Greefl). 

3.  The  amacrin  cells:  these  form  a  continuous  layer  in  the  innermost  portion  of  the 
inner  nuclear  layer.  Their  pear-shaped  body  measures  10  to  13.7  mu  and  gives  off 
a  single  process  inward.     One  finds: 

a)  Stratified  amacrin  cells:  these  have  only  one  process  ending  in  the  inner  plexi- 
form layer  with  a  superficially  parallel  branching. 

b)  Disseminated  amacrin  cells:  the  process  branches  many  times  and  ends  in  all 
parts  of  the  inner  plexiform  layer. 

c)  Association  amacrin  cells:  their  protoplasmic  processes  (dendrites)  end  in  the 
first  sublayer  of  the  inner  plexiform  layer,  the  axis  cylinder  process  courses  parallel 
to  the  surface  for  a  long  stretch  on  the  border  between  the  inner  nuclear  layer  and  the 
inner  plexiform  layer  and  breaks  up  into  numerous  branches.  These  cells  also  come  in 
contact  with  the  centrifugal  fibers. 

4.  The  already  reported  nuclei  of  Mueller's  supporting  fibers. 

Toward  the  fovea  the  thickness  of  the  inner  nuclear  layer  very  gradu- 
ally increases  and  attains  a  maximum  of  57  to  66  mu  between  the  wall 
about  the  fovea  and  its  center;  from  there  on  it  thins  out  very  rapidly 
and  practically  disappears  in  the  center  of  the  fovea.  Widely  isolated 
cells  are  often  seen  along  the  inner  surface  of  Henle's  fiber  layer  (PI.  V,  3). 

Although  the  layers  of  the  retina  are  entirely  without  vessels  as  far 
as  the  outer  plexiform  layer,  capillary  vessels  belonging  to  the  system  of 
the  arteria  centralis  retinae  are  found  even  in  the  inner  nuclear  layer  (PI. 
IV,  3,  c). 

6.      THE    INNER   PLEXIFORM   LAYER 
(PI.  IV,  3,  Ip) 

This  layer  shows  individual  variations  in  thickness  from  18  to  36  mu; 
in  some  instances,  however,  it  maintains  the  same  thickness  in  all  parts 
of  the  retina,  even  in  the  neighborhood  of  the  fovea.  It  is  wanting  only 
in  the  middle  of  the  fovea  and,  indeed,  in  a  somewhat  greater  expanse 
than  the  inner  nuclear  layer. 

It  possesses  a  finely  reticular  appearance,  like  the  inner  portion  of 
the  outer  plexiform  layer,  to  which  in  this  respect  it  is,  in  general,  similar, 
and  permits  several  secondary  or  sublayers  (usually  five)  to  be  recog- 
nized; these  are  darker  stripes  coursing  absolutely  parallel  to  the  sur- 
face of  the  retina  and  apparently  caused  by  a  thicker  interweaving  of 
the  fiber  mesh.     These  sublayers  arise  because  the  end  branches  of  the 


74  ANATOMY  AND  HlSTOLOCiY  OF  THE  HUMAN  EYEBALL 

nerve-cells,  coursing  parallel  to  the  surface,  only  lie  at  certain  levels  of 
the  inner  plexiform  layer.  The  nerve-cells  specially  concerned  in  the 
formation  of  the  sublayers  are  the  cone  bipolars  and  the  stratified  amacrin 
cells  of  the  inner  nuclear  layer  (along  with  their  proximal  processes), 
and  the  stratified  ganglion  cells  with  their  distal  i)rocesses.  In  general, 
the  sublayers  are  not  very  clearly  marked  in  the  human  retina;  in  the 
retina  of  the  birds  they  come  out  much  more  plainly. 

Although  the  structural  elements  of  the  inner  plexiform  layer,  includ- 
ing the  corresponding  parts  of  the  supporting  fibers,  are  without  nuclei, 
yet  this  layer  is  not  wholly  devoid  of  nuclei.  It  is  crossed  by  retinal 
vessels  and,  moreover,  other  isolated  displaced  cells  are  present  (ganglion 
or  amacrin  cells  ?). 

7.      THE    GANGLION-CELL   LAYER 
(PI.  IV,  3,  G) 

This  layer  has  a  thickness  of  10  to  20  mu  in  the  nasal  part  of  the 
retina  and  consists  of  a  single  row  of  ganglion  cells  separated  from  one 
another  by  Mueller's  fibers.  Besides  these,  neuroglia  cells  are  present. 
In  the  neighborhood  of  the  optic  nerve  the  ganglion  cells  form  a  closed 
row;  farther  toward  the  periphery  they  are  more  and  more  separated 
from  one  another,  and  the  spaces  are  filled  out  by  the  nerve-fiber  layer. 

Temporal  to  the  nerve  the  ganglion-cell  layer  is  somewhat  thicker 
and  the  cells  superimposed  in  two  layers.  This  superimposition  increases 
constantly  in  the  direction  of  the  fovea  until,  finally,  at  the  wall  of  the 
fovea  5  to  7  layers  of  ganglion  cells  are  present.  The  thickness  of  the 
whole  layer  is  thereby  increased  to  57-85  mu  on  the  nasal  side,  and  45-75 
mu  on  the  temporal  side  (Dimmer,  40). 

From  these  relations  in  thickness  it  follows  that  the  wall  of  the  fovea 
is  mainly  formed  by  the  ganglion-cell  layer.  From  here  on,  the  thickness 
of  the  layer  decreases  rapidly  toward  the  center  of  the  fovea,  and  it  is 
lost  or  fused  with  the  rudiment  of  the  nuclear  layer  while  still  in  the 
region  of  the  clivus  (PI.  V,  4). 

Since  the  same  relations  are  repeated  on  all  sides  of  the  fovea,  there 
arises  a  fairly  extensive  district  in  the  retina  in  which  the  ganglion  cells 
are  superimposed. 

This  area  (area  centralis,  Chievitz,  32)  has  about  the  same  extent 
as  the  other  fiber  layer  of  Henle,  i.e.,  a  circular  surface  area  of  about  4  mm 
radius;  from  the  fact  that  the  stratification  of  the  ganglion  cells  in  layers 
can  be  made  out  under  all  circumstances,  even  in  advanced  cadaverous 
changes,  we  have  here  a  sure  means  of  differentiating  the  temporal  from 
the  nasal  side  of  the  retina  and  the  section  need  not  necessarily  go 
through  the  fovea. 


THE  RETINA  75 

The  individual  ganglion  cells  show  a  most  varied  appearance.  In  the 
extrafoveal  portions  of  the  retina  very  large  ganglion  cells  with  a  diameter 
of  up  to  30  mu  occur;  their  nuclei  are  almost  exactly  round,  clear,  and 
10  to  II  mu  in  diameter.  The  nuclei  contain  large  shining  nucleoli. 
More  numerous  than  these  are  the  smaller  ganglion  cells  with  a  more 
oval  nucleus  of  8  to  9  mu  in  diameter  and  a  11  to  12  mu  protoplasmic 
body.  Only  the  smaller  forms  are  found  in  the  neighborhood  of  the 
fovea,  and,  corresponding  to  the  general  arrangement,  the  cell-body 
is  obliquely  elongated. 

The  ganglion  cells  are  multipolar  and  have  numerous  protoplasmic 
processes  (dendrites),  which  broaden  out  in  the  inner  plexiform  layer 
and  are  for  the  most  part  provided  with  axis  cylinders  going  over 
into  nerve-fibers  of  the  adjoining  layer.  The  protoplasm  contains  the 
so-called  Nissl  granules;  these  are  granules  and  shoals  of  varying  size,  of 
rounded  or  polyhedral  form,  giving  an  elective  stain  especially  with  blue 
dyes,  ordinary  and  polychrome  methyl  blue,  thionin,  etc.,  and  at  times 
also  with  hematoxylin.  The  granula  extend  into  the  protoplasmic 
processes  as  well,  but  not  into  the  cyhnders. 

The  peculiarities  of  the  Nissl  granula  in  man  are  little  known;  on 
technical  grounds  they  have  been  best  studied  in  lower  animals  (Bach, 
13;  Birch-Hirshfeld,  25;  Abelsdorff,  2;  only  the  latter  writer  depicts 
a  cell  in  man).  In  the  normal  retina  of  an  eye  enucleated  for  orbital 
carcinoma  which  I  was  able  to  study,  the  large  cells  contained  granula 
varying  much  in  size,  quite  uniformly  distributed  through  the  protoplasm, 
usually  with  a  granula-free  zone  immediately  about  the  nucleus;  the 
smaller  cells  contained  correspondingly  small  granula.  In  general,  it  is 
very  surprising  how  well  these  structures,  which  otherwise  are  very 
easily  disintegrated,  are  retained  in  detachment  of  the  retina  following 
traumatic  inflammation. 

Other  fixative  and  staining  methods  bring  out  a  fibrillar  structure 
in  the  cell-body  and  its  processes.  Dogiel  (44)  discovered  these  struc- 
tures with  his  methyl-blue  method;  then  Embden  (57)  demonstrated 
them  by  the  method  of  Bethe,  and  finally  Bartels  (16)  in  the  human 
with  the  method  of  Bielschofsky.  According  to  Bartels,  the  fibrillae 
course  from  one  process  to  another,  and  in  this  way  pass  by  the  cell-body; 
in  part,  too,  they  radiate  from  the  processes  toward  the  nucleus  and  possi- 
bly form  there  a  network.  The  fibrillae  are  extremely  fine  and  smooth 
(without  nodosities);  they  lie  more  loosely  in  the  protoplasmic  processes; 
in  the  axis  cylinder  processes  they  are  closely  pressed  together. 

According  to  Dogiel,  the  protoplasmic  processes  of  cells  of  the  same  tj-pe  are  united 
into  nets.     According  to  the  authors  named,  it  follows  from  the  fibrillar  structure 


76  ANATOMY  AND  HISTOLOGY  OF  THE  HUMAN  EYEBALL 

that  no  fundamental  ditTerence  exists  between  the  ])rot()i)hismic  processes  and  the  axis 
cylinder  processes;  both  arc  of  a  nervous  nature  and  the  fibrillae  are  genuine  conduct- 
ing organs. 

The  controversy  as  to  whether  the  ganglion  cells  have  a  granular  or  a  fibrillar  struc- 
ture appears  to  me  to  be  quite  useless;  both  structures  may  very  well  be  present  at  the 
same  time  and  only  one  or  the  other  come  out  after  a  certain  fixation.  A  view  which  in 
a  certain  sense  lies  between  these  two  extremes  is  that  of  Held  concerning  the  net-like 
structure  of  the  body  and  the  processes  of  the  nerve-cells  {Archiv.  fiir  Anatomie  und 
Physiologic,  anatomische  Abteilung,  1897,  p.  204),  yet  the  observations  of  this  investi- 
gator were  not  made  on  the  retina. 

The  Nissl  granules  cover  the  diplosome  of  the  ganglion  cell;  this  structure  is,  there- 
fore, only  visible  in  the  embryonal  eye,  before  the  development  of  the  granula  (cf.  chap, 
xvi). 

According  to  the  manner  in  which  the  dendrites  end  in  the  inner  plexiform  layer 
(wholly  analogous  to  the  amacrin  cells),  one  distinguishes  stratified  and  disseminated 
ganglion  cells.  The  former  spread  their  end  brushes  out  in  one  or  more  planes  of  the 
inner  ple.xiform  layer  and  in  this  way  produce  their  sublayers.  The  diffuse  ones 
branch  like  a  tree  and  end  everywhere  throughout  the  inner  plexiform  layer.  A  part 
of  the  cells  send  no  axis  cylinders  into  the  nerve-fiber  layer;  they  are  looked  upon  as 
displaced  amacrin  cells. 

The  neuroglia  cells,  which  we  meet  here  for  the  first  time  in  the  retina, 
have  smaller  and  more  densely-stained  nuclei  than  the  ganglion  cells, 
and  a  fiat  body.  Golgi  preparations  make  them  look  like  spider  cells 
here,  as  in  the  nerve-fiber  layer,  i.e.,  cells  provided  with  numerous  fine 
processes. 

8.      THE    NERVE-FIBER   LAYER 

(PI.  IV,  3,  yV) 

This  layer  is  thickest  (20  to  30  mu)  about  the  circumference  of  the 
optic-nerve  entrance,  and,  indeed,  about  the  upper,  nasal  and  lower 
portions. 

Toward  the  periphery  its  thickness  decreases,  rapidly  at  first,  then 
more  slowly.  In  the  extreme  periphery  the  nerve-fibers  and  the  ganglion- 
cell  layer  flow  together,  so  to  say,  i.e.,  the  few  ganglion  cells  still  present 
in  this  zone  lie  between  the  nerve-fibers  and  reach  to  the  basal  cones 
of  the  supporting  libers. 

At  the  temporal  border  of  the  optic  nerve  the  nerve-fiber  layer  is  very 
much  thinner  (some  1 1  mu) ;  its  thickness  decreases  still  farther  toward 
the  fovea;  it  goes  a  little  way  beyond  the  wall  of  the  fovea,  then  dis- 
appears entirely  (PI.  V,  4). 

Unlike  the  rest  of  the  layers  of  the  retina,  in  which  a  direction  parallel 
to  the  surface  or  a  plexus  formation  predominates,  the  nerve-fiber  layer 
shows  an  exquisite  fibrous  structure,  in  which  the  elements  are  in  general 
arranged  radial  to  the  optic-nerve  entrance.     In  the  entire  nasal  half  of  the 


THE  RETINA  77 

retina  this  convergence  toward  the  optic  nerve  is  not  disturbed;  in  the 
temporal  half,  however,  the  arrangement  of  the  fibers  varies  in  that  those 
fibers  which  pass  the  area  of  the  fovea  on  their  way  to  the  optic  nerve 
bow  away  from  it  and  go  around  it  in  circles  above  and  below  the  fovea. 
In  this  way  a  sort  of  raphe  arises  in  the  meridian  of  the  fovea  on  the 
temporal  side  in  the  periphery;  from  this  the  libers  go  off  like  feathers, 
above  and  below,  and  finally  at  the  temporal  border  of  the  fovea  itself 
they  form  an  actual  ring  (Greeff,  65).  Only  the  few  fibers  of  the  fovea 
itself  and  those  which  course  from  the  portions  lying  between  it  and  the 
optic  nerve  run  fairly  straight.  Since,  furthermore,  these  fibers  remain 
close  to  one  another  and  frequently  show  isolated  disease,  one  calls  this 
the  papillomacular  bundle;  the  loss  of  its  function  calls  forth  the 
appearance  of  typical  central  scotoma. 

The  fibrous  structure  of  the  nerve-fiber  layer  is  responsible  for  the  fact  that  extra- 
vasation into  this  layer  appears  to  be  made  up  of  fine  striae  or  short  streaks  arranged 
radial  to  the  optic  nerve.  E.xtravasates  in  the  other  layers  of  the  retina  appear  as 
rounded  flecks,  on  the  other  hand. 

The  nerve-fibers  in  the  retina  are  everywhere  grouped  in  small 
bundles;  these  unite  into  a  sort  of  net,  with  narrow  elongated  meshes. 
Rows  of  Mueller's  fibers  lie  in  these  meshes  and  the  rows  have  the  same 
direction  as  the  nerve-fibers.  The  nerve  bundles  are  spread  out  in  one 
plane,  as  a  general  thing;  the  nearer  one  approaches  the  optic  nerve, 
however,  the  more  slender  and  the  higher  the  bundles  become,  until  when 
very  close  to  the  optic  nerve,  they  become  superimposed  and  in  this  way 
go  over  into  the  grouping  characteristic  of  the  optic  nerve  itself. 

The  nerve-fiber  layer,  therefore,  presents  a  varying  picture  dependent 
upon  whether  the  section  runs  parallel  or  at  right  angles  to  the  fibers; 
on  longitudinal  section  the  nerve-fiber  layer  shows  a  fibrillation  parallel 
to  the  surface  and  only  shows  the  Mueller's  fibers  indistinctly;  the  cross- 
section,  on  the  other  hand,  shows  the  Mueller's  fibers  very  plainly,  and 
the  cross-sections  of  the  individual  small  nerve-fiber  bundles  lying  in  the 
arcades  formed  by  Mueller's  fibers  have  a  reticular  appearance.  One 
sees  the  picture  of  the  longitudinal  section  in  meridional  sections  and  the 
cross-section  picture  in  equatorial  sections.  As  a  result  of  the  unusual 
course  of  its  fibers,  the  region  of  the  fovea  makes  an  exception :  one  sees 
the  cross-section  picture  on  the  temporal  side  in  a  horizontal  section 
through  the  fovea;  on  the  nasal  side  of  the  fovea,  i.e.,  between  it  and  the 
papilla,  one  sees  the  longitudinal  section  picture.  A  vertical  section  of 
the  fovea  shows  the  cross-section  picture  on  each  side  of  the  depression. 

Aside  from  the  nerve-fibers  and  Mueller's  supporting  fibers,  the  layer 
in  question  also  contains  neuroglia.  As  in  the  optic  nerve  this  consists 
of  cells  and  fibers.     The  cells  have  a  longish,  quite  densely-staining 


78  ANATOMY  AND  IIIS'IOLOCiY  OF  THK  HUMAN  EYEBALL 

nucleus,  whose  axis  is  directed  parallel  to  the  course  of  the  nerve-fibers 
(PI.  IV,  3,  gl),  and  a  small  amount  of  protoplasm  of  varying  form.  The 
fibers  are  very  fine  and  form  a  meshwork  between  the  nerve-fibers.  (For 
more  details  concerning  the  neuroglia,  see  chap,  viii.) 

Finally,  the  nerve-fiber  layer  also  contains  the  larger  retinal  vessels, 
branches  of  the  arteria  and  vena  cenlralis  retinae.  They  are  imbedded  in 
the  nerve-fiber  layer  and  also  in  part  in  the  ganglion-cell  layer,  and  do  not 
bulge  the  inner  surface  of  the  retina  inward  at  all  or  not  appreciably  so. 
The  vessel  wall  is  relatively  little  developed,  especially  the  muscularis  of 
the  arteries;  the  adventitial  connective  tissue  is  sharply  set  off  against  the 
surrounding  nerve  tissue.  According  to  Kreuckmann  (123),  the  glial 
reticulum  ends  at  the  vessel  wall  in  a  sort  of  border  membrane,  which  he 
calls  the  limitans  perivascularis. 

These  tissues  (glia  and  vessel  wall),  which  have  originated  from  different  embryonal 
layers,  are  not  infrequently  separated  under  pathologic  conditions,  and,  for  instance, 
in  atrophic  conditions  of  the  retina  resulting  from  obliteration  of  the  vessels,  an 
interspace  is  formed  into  which  the  pigment  epithelium  frequently  grows;  the  well- 
known  picture  of  pigmentary  degeneration  of  the  retina  arises  in  this  way. 

The  modern  methods  of  study  (Dogiel's  methyl-blue  staining,  Golgi's  and  Ramon  y 
Cajal's  methods)  bring  to  light  still  other  details  not  to  be  recognized  in  ordinary  sec- 
tions (Greeff,  65).  The  nerve-fibers  are  clear,  non-medullated  fibers,  varying  from  an 
immeasurable  fineness  up  to  a  thickness  of  3-5  mu.  Divisions  of  the  fibers  occur  and 
among  the  typically  coursing  fibers  one  finds  individuals  which  cross  the  others. 

The  majority  of  the  fibers  come  from  the  ganglion  cells  of  the  retina,  and  are, 
therefore,  centripetal  and  serve  for  the  conduction  of  \'isual  sensations.  Aside  from 
these,  fibers  are,  however,  found  of  which  the  nerve-cells  do  not  lie  in  the  retina  and 
which,  therefore,  are  centrifugal  conductors.  According  to  Ramon  y  Cajal,  these 
are  larger  than  the  centripetal,  yet  they  course  in  the  nerve-fiber  layer  with  them.  If 
one  follows  such  a  fiber  in  a  centrifugal  direction,  one  sees  it  course  through  the  ganglion- 
cell  layer  and  the  inner  plexiform  layer  and  end  in  an  amacrin  cell,  by  means  of  a 
perivascular  nest,  and  at  other  amacrin  cells  by  processes.  These  fibers  do  not  press 
deeper  into  the  retina  than  the  layer  of  the  amacrin  cells. 

Moreover,  the  region  of  the  fovea  is  not  entirely  devoid  of  nerve-fibers.  A  deli- 
cate ring  is  found  at  the  place  where  the  nerve-fiber  layer  seems  to  cease  in  ordinary 
preparations,  according  to  Dogiel,  and  from  this  a  wide-meshed  ple.xus  of  fibers  extends 
over  the  floor  of  the  fovea. 

APPENDIX.      THE    SUPPORTING   FIBERS 
The  Radial  or  Mueller's  Fibers 

(PI.  IV,  3,  M) 

All  of  the  layers  of  the  retina  heretofore  discussed  contain  a  frame- 
work which  is  not  of  a  nervous  nature,  i.e.,  it  does  not  serve  either  for  the 
perception  or  conduction  of  light  (in  a  centripetal  or  centrifugal  direction). 
This  framework  consists  of  elongated  cells,  the  radial  or  Mueller's  sup- 


THE  RETINA  79 

porting  fibers  already  repeatedly  mentioned.  They  course  through  the 
extrafoveal  portions  of  the  retina  parallel  to 'the  surface;  starting  along 
the  inner  surface  of  the  retina  in  conical  expansions  (closely  placed  bases), 
they  form  quite  closely  placed  rows  of  small  nerve-fiber  bundles  in  the 
nerve-fiber  layer,  diverge  and  break  up  beyond  the  ganglion-cell  layer. 
From  the  inner  nuclear  layer  on,  the  fibers  lie  isolated  and  are  uniformly 
distributed. 

They  participate  especially  in  the  formation  of  the  plexiform  layers 
in  that  they  give  ofif  numerous  fine  extensions  parallel  to  the  surface  in 
the  level  of  these  layers.  Furthermore,  they  form  the  reticulum  of  the 
nuclear  layers  by  means  of  wing-like  processes,  go  over  into  the  memhrana 
limitans  externa,  and  finally  end  between  the  inner  members  of  the  rods 
and  cones  in  the  above-reported  fiber-baskets  by  means  of  fine  fibrillae. 
Their  cell  nature  is  shown  by  a  longish  nucleus  lying  in  a  mid-level  of  the 
inner  nuclear  layer. 

Since  the  Mueller's  fibers  possess  no  processes  or  branchings  (or  only  a 
few)  in  the  territory  of  the  nerve-fiber  and  ganglion-cell  layers,  they  are 
much  more  plainly  set  off  from  the  surrounding  tissues  in  these  layers 
and  are  to  be  seen  without  special  aid  in  ordinary  preparations,  especially 
when  the  section  falls  at  right  angles  to  the  course  of  the  nerve-fibers, 
because  one  then  has  the  rows  of  fibers  before  him  in  profile  view.  In 
the  rest  of  the  layers  they  are  not  to  be  made  out  so  off-hand ;  they  and 
their  relations  to  these  layers  come  out  plainly  by  the  method  of  Ramon 
y  Cajal  or  Held's  stain. 

The  nerve-fibers  do  not  go  through  the  basal  cones,  as  stated  by  Greeff , 
but  between  them. 

In  the  neighborhood  of  the  fovea,  the  superficially  perpendicular 
course  of  the  supporting  fibers  present  in  the  extrafoveal  territory  of  the 
retina  (perpendicular  to  the  surface)  gives  way  to  an  increasing  obliquity 
and,  indeed,  in  the  same  sense  as  the  clivus  does,  but  it  does  not  exactly 
copy  this  declivity.  The  bending  away  from  the  surface  is  most  marked 
in  the  region  of  the  outer  fiber  layer — in  the  rest  of  the  layers  it  is  less  so, 
so  that,  in  general,  an  5-form  curve  is  brought  about. 

9.      MEMBRANA   LIMITANS   INTERNA 
(PL  IV,  3,  Li) 

In  the  matter  of  the  definition  of  this  tenn  two  \news  are  still  opposed,  as  they  were 
in  earlier  periods.  The  one  holds  the  membrana  limitans  interna  to  be  the  inner  limita- 
tion of  the  retina  formed  by  the  uninterrupted  apposition  of  the  basal  cones  of  Mueller's 
sujjporting  fibers  and  logically  considers  the  anatomically  demonstrable  membrane 
lying  inside  the  basal  cones  to  be  the  border  membrane  of  the  \-itreous,  and  calls  it  the 
membrana  hyaloidea. 


8o  ANATOMY  AND  HISTOLOGY  OF  THE  HUMAN  EYEBALL 

The  other  calls  the  latter  the  membrana  Umilans  interna  retinae  and,  therefore, 
denies  the  existence  of  a  hyaloidea. 

Against  the  first  conception  it  is  to  be  argued  that  a  limitation  of  the  retina  is 
probably  effected  by  the  totality  of  the  basal  cones  of  the  supporting  fibers,  but  only 
so  in  the  sense  that  it  is  a  mathematical  surface,  not  an  actual  border  membrane.  The 
second  definition  of  the  m.  Umitans  interna  has  for  it  that  this  membrane  is  anatomically 
demonstrable,  that  is,  has  a  measurable  thicluiess  and  all  the  properties  of  a  glass 
membrane.  In  any  case,  one  must  say  that  this  membrane  has  just  as  much  relation 
to  the  vitreous  as  it  has  to  the  retina,  and  that  it  looks  like  the  inner  glass  membrane 
of  the  retina  in  one  preparation  and  like  the  outer  border  membrane  of  the  vitreous 
in  another. 

How  Tornatola  (224)  comes  to  deny  the  existence  of  a  membrane  between  the 
retina  and  vitreous  altogether  cannot  be  understood  without  seeing  the  preparations 
concerned.  I  have  seen  the  membrane  in  many  hundreds  of  eyes  under  normal  as  well 
as  under  the  most  varied  pathologic  conditions,  and  so  has  everyone  else  seen  it. 
There  has  only  been  contention  concerning  what  it  should  be  called  and  to  what  it 
belonged.  I  myself  (184)  have  in  my  time  called  it  the  hyaloidea,  following  Retzius, 
but  I  now  prefer  the  name  Umitans  interna,  because  it  seems  to  me  that  the  majority 
of  authors  so  designate  it,  and  because  an  analogous  membrane  is  present  on  the  inner 
surface  of  the  ciliary  epithelium. 

So  far  as  has  yet  been  described,  there  lies  between  the  retina  and  the 
vitreous  one  (and  only  one)  anatomically  demonstrable  membrane,  the 
membrana  Umitans  interna  retinae.  It  is  a  glass  membrane  of  i  to  2  mu 
thickness,  which  in  surface  view  sometimes  shows  the  impressions  of  the 
basal  cones  of  Mueller's  fibers  as  irregular  polygonal  fields.  It  continues 
uninterrupted  over  th.e  fovea  centralis,  without  any  essential  change;  it  is, 
on  the  other  hand,  gradually  lost  at  the  optic-nerve  entrance  and  also  possi- 
bly in  many  eyes  at  the  ora  serrata.     (For  more  details  see  chap,  ix,  8.) 

b)  Histologic  and  Functional  Divisions  of  the  Retina.     Its  Blood- 
vessels and  Fovea  Centralis 

Following  the  detailed  description  of  the  structure  of  the  retina,  a 
short  survey  of  the  significance  of  the  individual  elements  and  their 
reciprocal  relations  is  in  order.  In  anticipation  of  some  details  of  develop- 
mental history  of  the  eyeball,  one  may  distinguish  between  elements  of 
ectodermal  and  of  mesodermal  origin  in  the  retina. 

The  elements  of  ectodermal  origin  are  partly  framework  elements, 
partly  nerve-cells. 

The  framework  elements  are  partly  of  the  same  nature  as  the  retina 
proper  (Mueller's  supporting  fibers  and  the  two  membranae  limitantes, 
partly  of  the  same  nature  as  in  the  optic  nerve  (neuroglia).  The  latter 
is,  however,  found  only  in  those  layers  which  represent  a  direct  expansion 
of  the  optic  nerve. 


THE  RETINA  Si 

The  nerve-cells  of  the  retina  are  grouped  in  three  superimposed  planes. 
Just  how  these  planes  are  united  opinions  differ.  A  majority  of  the 
authors  hold  firmly  to  the  view  of  Ramon  y  Cajal  that  the  cells  subse- 
quent to  one  another  (in  the  sense  of  nerve  conduction)  (the  neurons)  are 
not  united  but  only  lie  in  contact  with  one  another,  or  are  surrounded  by 
their  branches.  Another  view,  espoused  by  Apathy  and  Bethe,  looks 
upon  the  neurofibrilla  as  the  essential  structural  element  of  the  nervous 
system  and  conceives  of  a  continuity  of  the  neurofibrillae. 

It  is  certain  that  the  individual  layers  possess  a  degree  of  independence 
of  one  another,  e.g.,  atrophic  processes  are  often  limited  to  one  plane. 
In  any  case  the  neuron  theory  of  Ramon  y  Cajal  contributes  more  to  the 
understanding  of  these  conditions  than  does  the  neurofibrillar  theory. 

The  retina  contains  three  neurons;  they  are  named  in  the  sense  of 
their  conduction,  therefore,  in  this  case  in  the  centripetal  direction. 

The  first  neuron  is  represented  by  the  rods  and  cones  and  the  outer 
nuclei  belonging  to  them.  Its  elements  are  broadened  out  into  a  simple 
surface  layer;  only  the  nuclear  portions  are  superimposed  in  layers, 
because  of  their  greater  volume.  This  neuron  serves  for  the  reception 
of  the  individual  light  impressions  and  the  mosaic  arrangement  of  its  ele- 
ments makes  possible  a  separation  of  the  respective  impressions  in  space 
and  the  appreciation  of  the  picture  projected  through  the  optical  system. 
This  is  the  sensory  epithelium  (neuroepithelium)  of  the  retina  (Schwalbe). 
In  the  matter  of  its  nutrition  the  first  neuron  is  entirely  dependent  upon 
the  choriocapillaris,  because  it  contains  no  blood-vessels.  It  therefore 
happens  that  a  circumscribed  atrophy  of  the  choriocapillaris  leads  to  a 
coextensive  atrophy  of  the  pigment  epithelium  and  of  the  first  neuron  of 
the  retina,  so  that  the  subjective  functional  defect  (the  scotoma)  and 
the  objective  ophthalmoscopic  change  (the  atrophic  area)  exactly  corre- 
spond in  this  instance. 

The  second  neuron  is  represented  by  the  nerve-cells  of  the  inner 
nuclear  layer.  These  cells  come  in  contact  with  several  elements  of  the 
first  neuron.  However,  there  are  cells  here  which  serve  exclusively  for  the 
union  of  the  elements  with  one  another.  Finally,  centrifugal  nerve-fibers 
come  into  contact  with  individual  elements  of  this  neuron.  It  is  to  be 
conceived  that  nervous  processes,  even  of  a  high  order,  take  place  in 
this  la3'er. 

The  third  neuron  is  formed  by  the  ganglion  cells;  it  is  the  longest  of 
all,  for  its  axis  cylinders  reach  through  the  optic  nerve,  the  chiasm  and 
tractiis  opticus  as  far  as  the  brain  (outer  geniculate  body,  optic  thalamus 
and  anterior  corpora  quadrigemina,  Bernheimer,  23). 

The  second  and  third  neurons  possess  a  vessel  system  of  their  own  in 


82  ANATOMY  AND  HISTOLOGY  OF  THE  HUMAN  EYEBALL 

the  branches  of  the  artcria  centralis  retinae  and  the  veins  of  the  same  name. 
These  are  the  only  elements  of  mesodermal  origin  in  the  retina.  The 
largest  branches  He  superficially  in  the  nerve-fiber  layer,  yet  the  ganglion- 
cell  layers  are  usually  absent  beneath  them  and  even  the  inner  nuclear 
layer  shows  a  thinning;  on  the  other  hand,  the  inner  surface  of  the  retina 
is  bulged  slightly  forward.  The  finer  branches  pass  deeper  into  the  nerve- 
fiber  layer  and  the  most  extensive  branching  is  found  in  the  ganglion-cell 
layer. 

According  to  His  (105),  the  capillary  net  of  the  retina  consists  of 
narrow  capillaries  (5  to  6  mu,  seldom  wider),  and  is,  naturally,  most 
developed  in  the  posterior  segment  of  the  retina.  Fine  branches  (art. 
afferantes)  are  here  given  off  at  right  angles  from  place  to  place;  these 
first  break  up  into  capillaries  some  0.13  to  0.25  mm  from  the  main 
vessels.  In  this  way  a  space  free  from  capillaries  arises  on  each  side  of 
the  main  vessel.  This  first  arterial  capillary  net  lies  in  the  nerve-fiber 
layer;  from  it  there  ascend  branches  to  the  inner  nuclear  layer  and  each 
of  these  forms  a  venous  capillary  net  on  the  outer  and  inner  surface  of  this 
layer.  The  venous  radicals  (venae  efferentes)  then  form  from  these, 
descend  again  to  the  nerve-fiber  layer,  and  empty  into  larger  veins  at 
right  angles. 

The  capillaries  go  as  far  as  the  outer  surface  of  the  inner  nuclear  layer 
and  no  farther.  The  border  between  the  first  and  second  neuron  (PI. 
IV,  3,  x)  is,  therefore,  also  the  border  between  the  avascular  portion  of 
the  retina  (the  sensory  epithelium)  and  the  vascular  portion  (cerebral 
layers  of  Schwalbe). 

Toward  the  ora  serrata  the  vessel  net  becomes  more  and  more  simple. 
The  peripheral  limit  of  the  retinal  vessel  system  does  not,  however, 
coincide  with  the  border  of  the  retina,  but  lies  about  i  mm  farther  back; 
a  few  wide  irregular  projected  loops  form  the  border  of  the  retinal  vessel 
system.  These  loops  have  a  somewhat  wider  caliber  than  the  rest  of 
the  capillaries,  and,  if  one  so  wishes,  one  may  speak  of  a  direct  transition 
of  arteries  into  veins  here.  I  have  not  been  able  to  make  out  an  out- 
spoken circular  course  of  the  terminal  venous  branches  in  my  preparations. 

The  fovea  centralis,  of  which  the  gross  anatomic  relations  have  been 
described  above,  arises  mainly  from  a  spreading  apart  of  the  cerebral 
layers.  For,  aside  from  isolated  cells  of  the  second  neuron  and  the  very 
sparse  nerve-fibers  found  on  the  floor  of  the  fovea,  this  area  contains  only 
elements  of  the  first  neuron.  Along  with  the  cerebral  layers,  the  retinal 
vessels  also  fail  in  the  center  of  the  fovea.  A  capillary-free  area  of  0.4 
to  o .  5  mm  size  and  irregular  form  is  present  in  most  cases.  Yet  this 
area  varies  appreciably,  probably  with  the  size  of  the  fovea  in  general. 


THE  RETINA  83 

With  the  ophthalmoscope  this  vessel-free  area  appears  larger,  because  a 
magnification  of  only  14  times  in  the  direct  image  does  not  permit  the 
capillaries  to  be  seen;  the  finest  ophthalmoscopically  visible  vessels 
extend  only  a  little  way  over  the  wall  about  the  depression  in  the  retina 
indicated  by  the  macular  reflex. 

Since  the  histologic  relations  of  the  fovea  centralis  have  already  been 
described  somewhat  more  accurately  in  connection  with  the  indi\-idual 
layers,  it  seems  superfluous  to  give  a  special  histologic  description  of  the 
fovea.  Still,  certain  peculiarities  of  the  retina  in  this  region  may  well  be 
again  briefly  pointed  out.  A  circle,  tangent  to  the  temporal  border  of 
the  disc,  drawn  about  the  center  of  the  fovea  contains  in  its  compass 
(the  area  centralis)  two  remarkable  histologic  findings:  the  thickening 
of  the  ganglion-cell  layer  and  the  outer  fiber  layer  of  Henle.  The  area, 
naturally,  has  no  sharp  limits;  therefore,  its  size  can  only  be  approxi- 
mately given.  The  wall  about  the  fovea  is  formed  by  the  ganglion-cell 
layer.  In  the  middle  of  the  fovea  is  an  area  scarcely  of  o .  5  mm,  char- 
acterized by  extreme  attenuation  of  the  cerebral  layer  and  by  absence  of 
the  retinal  vessels,  as  well  as  by  the  presence,  exclusively,  of  cones.  But 
the  slender  foveal  cones  are  found  only  in  the  very  center  of  this  area. 

Here  everything  is  arranged  to  increase  functional  capacity  as  much 
as  possible.  The  fovea  lies  in  the  optic  axis  (although  not  exactly  so), 
where  the  picture  projected  by  the  optical  system  is  the  sharpest.  The 
inner  layers  of  the  retina  are  spread  apart,  for  they  can  only  obscure 
the  image;  the  shadow-producing  retinal  vessels  fail  entirely.  Only  the 
elements  of  higher  dignity,  the  color-perceiving  cones,  are  present,  and 
these,  moreover,  possess  a  fineness  found  in  no  other  part  of  the  retina. 
Each  cone  is  united  with  but  a  single  bipolar  cell  and,  furthermore,  with 
but  one  ganglion  cell  possibly,  so  each  individual  light  sensation  is  con- 
ducted isolated  from  the  rest.  The  center  of  the  fovea  centralis  thus 
becomes  the  seat  of  the  highest  function  of  the  eye — that  of  central  vision. 

The  yellow  color  of  the  macula  liitea  is  conditioned  by  a  coloring  mat- 
ter distributed  through  all  the  elements  of  the  cerebral  layer,  although 
through  the  outer  fiber  layer  to  a  lesser  degree  (Dimmer,  40). 

c)    The  Extreme  Periphery  of  the  Retina 

When  the  retina  is  viewed  from  its  surface,  its  (anterior)  border  (era 
serrata  retinae)  appears  more  or  less  toothed  (in  biologic  terminology), 
i.e.,  sharp  projections  are  directed  toward  the  corona  ciliaris  and  sepa- 
rated from  one  another  by  rounded  bays.  The  length  of  the  teeth  is 
subject  to  great  variation;  in  their  maximal  development  they  may 
reach  to  the  corona  ciliaris. 


84  ANATOMY  AND  HISTOl.OGY  OF  THE  HUMAN  EYEBALL 

As  a  rule  the  teeth  arc  not  uniformly  developed  in  the  entire  circum- 
ference of  the  border  of  the  retina,  but  arc  plainer  on  those  sides  where 
the  ciliary  body  is  shorter,  i.e.,  especially,  therefore,  on  the  nasal  side 
(PL  II,  i);  on  the  temporal  side  they  often  fail  completely,  and  the 
border  is  then  only  finely  and  irregularly  wavy  or  angular. 

The  teeth  correspond  in  position  to  the  intervals  between  the  ciliary 
processes  and  all  the  irregularities  of  development  in  the  corona  ciliaris 
are  reflected  in  the  ora  serrata.  Depending  upon  their  length,  the  form 
of  the  teeth  varies  from  that  of  a  triangle  to  an  awl ;  the  bays  are  rounded 
out  as  a  whole,  but  often  show  several  smaller  projections  and  in  this  way 
acquire  an  undulating  or  slightly  jagged  outline. 

That  which  appears  to  be  the  border  of  the  retina  in  a  surface  prepara- 
tion by  no  means  coincides  with  the  corresponding  border  of  the  pigment 
epithelium,  for  the  border  of  the  retina  usually  lies  farther  forward 
(cf.  p.  62).  One  can  get  the  best  general  view  of  the  topographic  rela- 
tions of  these  two  borders  after  the  retina  has  been  detached  from  the 
uvea  in  a  hardened  eye;  the  pigment  epithelium  of  the  chorioidea 
(posterior  zone  of  the  pigment  epithelium)  remains  with  this,  but  the  pig- 
ment epithelium  of  the  ciliary  body  remains  attached  to  the  retina,  at 
least  far  enough  so  that  one  can  recognize  its  limits.  The  finer  details 
vary  greatly.  Schoen  (193)  differentiates  four  types,  yet  it  is  not  always 
an  individual  difference  alone,  for  the  relations  in  different  parts  of  the 
same  eye  are  not  always  the  same. 

In  many  eyes  one  finds  peculiar  cavities  in  the  tissues  (Blessig's 
cysts,  Iwanoff's  retinal  oedema,  cystoid  degeneration)  in  the  most  periph- 
eral portions  of  the  retina.  On  surface  view  (PI.  V,  5)  they  appear  as 
rounded  pores,  or,  through  confluence  of  adjoining  cavities,  as  lobulated 
or  meandering  or  dendritically  branched  clear  flecks,  often  separated  by  a 
narrow  partition  only. 

The  first  traces  appear  very  early  in  life,  between  perhaps  the  years  of 
16  and  20,  and  at  first  immediately  behind  the  teeth  of  the  ora  serrata. 
From  here  the  cavities  broaden  out  backward  and  toward  the  sides,  and 
the  originally  isolated  flecks  flow  into  a  closed  zone,  which  always  becomes 
broader  with  the  years.  But  the  cavities  increase  not  only  in  extent  but 
also  in  size ;  the  individual  cavities  enlarge  and  so  merge  with  one  another 
in  age  that  only  column-like  pieces  of  the  separating  walls  remain. 

Cystoid  degeneration  is,  therefore,  probably  to  be  considered  as 
physiologic  despite  the  great  individual  variations  in  grade  and  extent 
of  the  change;  it  increases  with  age,  somewhat  as  the  far  point  always 
moves  away  with  the  years.     The  cavities  are  not,  however,  peculiarly 


THE  RETINA  85 

senile  appearances,  for  one  may  encounter  them  in  beautiful  formation 
even  between  the  ages  of  30  and  40. 

The  thickness  of  the  retina  is  markedly  decreased  by  the  formation 
of  cavities.  In  such  eyes  the  minimum  thickness  is  not  just  at  the  border 
of  the  retina  but  behind  the  zone  of  the  cavities  (PI.  V,  6). 

Great  difficulties  lie  in  the  way  of  one  who  wishes  to  produce  a  picture  of  the 
histologic  structure  of  the  extreme  periphery  of  the  retina,  for  light  pathologic  changes 
can  occur  in  this  region  which  entirely  escape  the  control  of  clinical  observation. 

The  extreme  periphery  of  the  retina  is  not  accessible  to  ordinary  ophthalmoscopic 
study.  This  region  is  only  visible  to  the  ophthalmoscope  under  especially  favorable 
circumstances,  such  as  coloboma  of  the  iris  with  aphakia  (Reimar,  178),  or  in  tuniors 
which  press  the  ora  serrata  toward  the  optic  axis.  Traptas  (225)  has  given  a  method 
(pressing  in  the  wall  of  the  bulb  with  the  finger),  but  we  still  lack  a  comprehensive 
investigation  of  the  ophthalmoscopic  appearance  of  this  region,  and  especially  a  control 
of  these  findings  through  anatomy. 

In  addition,  it  happens  that  the  most  peripheral  portions  of  the  retina  are  blind 
(Bonders,  45).  The  visual  field,  as  well  known,  has  an  extent  of  only  60°  at  the  most 
on  the  nasal  side;  on  the  temporal  side  it  has  one  of  90°  or  even  more.  If  one  construct 
this  angle  in  the  schematically  cross-sectioned  eye  (Text  Fig.  i)  with  one  side  along  the 
visual  axis  and  the  apex  at  the  posterior  nodal  point  (which  falls  at  about  the  posterior 
pole  of  the  lens),  one  notes  that  a  4  mm  broad  zone  of  the  retina,  at  least  on  the 
temporal  side,  is  devoid  of  a  visual  function.  When  a  pathologic  condition  develops 
in  this  zone  it  is,  therefore,  neither  appreciable  objectively  (by  the  ophthalmoscope)  nor 
subjectively  (by  testing  the  function). 

Yet  the  appearance  of  the  outermost  periphery  of  the  retina  is  altered  by  age,  alone. 
It  is,  therefore,  fundamentally  incorrect  to  base  the  description  upon  the  relations  in 
children's  eyes  only,  for  then  all  later  changes  are  classified  as  pathologic,  which  most 
certainly  is  not  correct. 

The  fact  that  the  outermost  periphery  of  the  retina  is  blind  should 
cause  a  high  degree  of  astonishment,  because  the  histologic  structure  of 
the  retina  does  not  justify  this  observation.  The  organs  for  the  reception 
of  light-stimuli  (the  rods  and  cones)  reach,  indeed,  to  the  border  of  the 
retina;  the  elements  simply  spread  apart  somewhat,  becoming  thicker  and 
shorter;  therewith  the  number  of  the  rods  decreases  much  more  than  the 
cones  (Greeff,  75).  With  the  cessation  of  the  rod-and-cone  layer  the 
limitans  externa  bends  down  toward  the  pigment  epithelium  and,  accord- 
ing to  Wolfrum  (242),  goes  over  into  the  cement  ridges  lying  between 
the  pigment  epithelium  and  the  non-pigmented  ciliary  epithelium.  The 
anterior  border  of  the  limitans  externa  coincides  exactly  with  the  border 
in  the  pigment  epithelium. 

The  two  nuclear  layers  become  correspondingly  thinner  toward  the 
border  of  the  retina,  but  otherwise  show  no  striking  changes.  At  the 
same  time  nuclei  (outwardly  displaced  inner  nuclei  ?)  appear  in  the  outer 


86  ANATOMY  AND  HlSTOl.OGV  OF  THK  HUMAN  EYEBALL 

plexiform  layer;  the  border  line  between  the  two  nuclear  layers  thereby 
becomes  indistinct,  and  finally  they  fuse  into  one  at  the  border  itself. 

No  changes  can  be  made  out  in  the  inner  nuclear  layer.  The  ganglion- 
cell  layer,  together  with  the  nerve-fiber  layer,  cease  0.5  to  i  mm  behind 
the  ora  serrata.  To  the  same  extent  in  which  these  elements  disappear, 
the  supporting  tissue  increases  in  mass;  the  outermost  periphery  of  the 
retina,  therefore,  contains  only  scattered  neuroglia  cells  in  place  of 
ganglion  cells,  only  closely  pressed  basal  cones  of  Mueller's  fibers  in  place 
of  nerve-fibers,  and,  therefore,  looks  cross-  or  diagonally  striated.  The 
Umitaus  interna  retinae  often  becomes  notably  thinner,  and,  so,  often 
indistinct  toward  the  ora  serrata  (cf.  chap,  ix,  8). 

At  the  border  itself,  the  retina  of  the  adult  (and  the  description  relates 
only  to  such)  is  sharply  set  off  against  the  ciliary  epithelium  (PI.  V,  6). 
Since  now  this  is  much  thinner  than  the  retina,  there  arises  a  step  at 
the  border  of  the  retina,  sometimes  rounded,  sometimes  sharply  angular, 
sometimes  falling  abruptly  at  right  angles,  sometimes  overhanging  the 
ciliary  epithelium.  Not  only  are  individual  variations  found  here,  but 
the  form  of  the  border  of  the  retina  changes  in  different  sections  from  the 
same  eye.  For  example,  the  more  the  section  approaches  the  apex  of  a 
tooth  of  the  ora  serrata,  the  more  the  border  of  the  retina  overhangs.  In 
such  sections  one  sees  a  sort  of  spur  made  up  of  a  loose  reticulum  with  a 
few  irregularly  arranged  nuclei  projecting  into  the  vitreous  from  the  inner 
surface  of  the  retina.  When  the  section  goes  exactly  through  the  front 
of  a  tooth,  this  spur  lies  upon  the  inner  surface  of  the  ciliary  epithelium. 
Usually  there  rules  a  relationship  similar  to  that  of  the  pterygium  corneae, 
which  also  is  grown  fast  along  the  middle  line  while  the  marginal  portions 
are  undermined.  This  comparison  should,  however,  only  serve  for  the 
visualization  of  the  relations;  I  am  far  from  the  intention  of  conceiving 
of  the  origin  of  the  ora  serrata  as  similar  to  that  of  the  pterygium.  One 
now  and  then  sees,  rather,  a  little  group  of  elongated,  fibrillated 
ciliary  epithelium  cells  at  the  border  of  the  retina,  after  the  manner 
of  a  buttress. 

The  first  traces  of  cystoid  degeneration  are  shown  in  a  rarification  of 
the  interior  of  the  retinal  tissue  close  to  the  border  and  its  transformation 
into  a  widespread  reticulum.  The  formation  of  actual  smooth-walled 
cavities,  mostly  rounded  in  form,  takes  place  later.  But  numerous  small 
isolated  cystic  spaces  are  always  found  at  the  posterior  border  of 
the  degenerated  zone  in  the  far-advanced  cystic  degeneration  of  older 
people. 

Such  young  cavities,  so  to  say,  lie  in  the  outer  plexiform  layer,  and 
soon  extend  from  there  into  the  outer  nuclear  layer,  often  more  into  the 


THE  RETINA  87 

inner  nuclear  layers;  they  are  rounded  or  oval,  sharply  bordered,  mostly 
empty,  and  more  rarely  divided  by  remnants  of  the  outer  plexiform  layer 
into  planes.  The  immediate  neighborhood  of  the  cavities  shows  no  strik- 
ing changes. 

The  larger  cavities  (PI.  V,  7)  reach  so  close  to  the  »i.  limitans  externa 
that  only  a  layer  of  outer  nuclei  remains;  indeed,  even  this  may  dis- 
appear. The  larger  spaces  extend  inward  as  far  as  the  inner  fiber  layer, 
yet  a  covering  made  up  of  remnants  of  the  nerve-fiber  layer  and  the  m. 
limitans  retina  almost  always  remains.  The  tissue  intervening  between 
the  spaces,  which  more  and  more  back  up  against  each  other,  thickens 
itself  into  partitioning  walls  made  up  of  tensely  spanned  protoplasmic 
fibers  provided  with  longish  nuclei  between  which  lie  the  spaced-apart 
outer  nuclei.  This  is  associated  with  a  very  considerable  increase  in  the 
thickness  of  the  retina — to  almost  double  the  original  thickness. 

The  highest  grade  of  cavity  formation  consists  in  a  further  reduction  of 
the  partitioning  walls  to  individual  columns  and  extensive  confluence 
of  the  cavities.  Further  histologic  changes  do  not  appear.  This  highest 
grade  is  better  recognized  in  surface  preparations  of  the  entire  retina 
than  in  individual  sections.  The  cavities,  however,  never  break  through 
the  limits  set  for  them  by  the  two  m.  limitantcs;  even  in  their  most 
extensive  development  they  remain  intraretinal. 

I  should  consider  the  following  to  be  pathologic  changes:  complete 
absence  of  rods  and  cones  in  the  outermost  periphery  and  the  fixation  of 
the  limitans  externa  to  the  pigment  epithelium,  as  well  as  the  occasional 
occurrence  of  high-grade  thinning  of  the  peripheral  portions  of  the  retina, 
with  loss  of  the  regular  stratification  without  cavity  formation. 

The  peculiar  morphologic  relations  of  the  ora  serrala  have  been  first  thoroughly 
studied  in  the  last  decades  and  various  theories  concerning  their  origin  have  been  put 
forth. 

According  to  Schoen  (193),  in  the  newborn  the  border  of  the  retina  shows  a  straight 
course  and  only  microscopic  little  juttings,  some  850  in  number,  are  present.  He  holds 
this  condition  to  be  the  normal  one;  the  larger  microscopicaUy  visible  juts  first  appear 
later;  they  are,  therefore,  acquired  and  are,  for  Schoen,  equivalent  to  a  pathologic 
condition.  The  cause  of  this  diseased  condition  is  the  pull  of  the  zonular  fibers  attached 
to  the  juts,  and  this  again  arises  from  overstraining  of  the  accommodation. 

It  is  not  difficult  to  disprove  this  theorj-,  because  the  premises  are  not  warranted. 
E.  von  Hippel  (102)  found  larger  teeth  at  the  ora  serrala  in  the  newborn.  The  zonular 
fibers  are  not,  in  general,  inserted  directly  into  the  border  of  the  retina,  and  even  if  this 
were  so,  the  pull  of  the  zonula  could  not  bring  about  the  jutting  form  but  only  a  uni- 
form displacement  of  the  border  of  the  retina,  because  zonular  fibers  are  also  richly 
present  in  the  situations  corresponding  to  the  bays  of  the  ora  serrala. 

O.  Schultze  (198),  on  the  other  hand,  proceeds  from  the  development  of  the 
retina  and  the  cihary  epithelium.     In   the  fourth  month  of  pregnancy  the  border 


88  ANATOMY  AND  HISTOLOGY  OF  THE  HU:MAX  EYEBALL 

between  the  two  lies  just  behind  the  ciliary  processes  and  extends  into  the  ciliary 
valleys  by  broad,  short  projections.  In  further  development,  the  retinal  border  con- 
stantly moves  backward  behind  the  processes,  and  the  teeth  become  more  and  more 
drawn  out.  These,  too,  on  their  part  now  undergo  more  or  less  regression  and  leave 
behind  more  markedly  pigmented  stripes,  the  striae  ciliares  (cf.  chap.  i.x). 

This  theory  is  not,  indeed,  entirely  satisfactory,  but  has  much  more  in  its  favor 
than  that  of  Schoen.  It  is  exactly  the  opposite  of  this.  Whereas  Schoen  has  the  ora 
serrala  develop  in  extrauterine  life,  O.  Schultze  considers  it  as  congenital  and  admits, 
indeed,  the  possibility  of  later  regression. 


CHAPTER  VIII.     THE  OPTIC  XERVE  (XERVUS  OPTICUS) 

Besides  the  intraocular,  only  the  orbital  portion  of  this  nerve,  which 
represents  the  union  of  the  retina  with  the  brain,  will  be  described. 

By  the  intraocular  or  bulbar  portion  of  the  optic  nerve,  one  under- 
stands the  part  lying  within  the  bulb  wall,  that  portion  remaining  behind 
on  the  bulb  after  a  properly  made  enucleation,  i.e.,  when  the  optic  nerve 
is  cut  through  at  the  level  of  the  outer  surface  of  the  sclera.  The  orbital 
or  retrobulbar  portion  is  that  lying  between  the  eyeball  and  the  canalis 
opticus. 

The  division  into  a  non-medullated  and  a  medullated  section  is  better 
than  this  division.  The  border  between  these  two  sections  falls  pretty 
nearly  at  the  outer  surface  of  the  lamina  cribrosa,  and  still  lies,  therefore, 
within  the  bulbar  portion.  This  limit  is,  of  course,  not  an  absolutely 
sharp  one,  for  the  medullary  sheaths  of  nerve-fibers  do  not  all  cease  at  the 
same  place,  but  it  has  the  adv^antage  of  being  a  natural  limit,  one  which 
bears  the  most  important  differences  in  structure. 

a)    The   Non-medullated  Portion  of  the  Optic  Nerve 
(PL  IV,  4) 

I.      MICROSCOPIC   ANATOMY   AXD   HISTOLOGY 

This  portion  can  be  further  subdivided  into  a  retinal,  chorioidal  and 
a  scleral  portion.  These  portions  correspond  to  the  like-named  coats  of 
the  bulb  wall  through  which,  one  after  another,  the  optic  nerve  passes. 

When  one  foUows  the  nerve-fiber  layer  in  a  centripetal  direction  in  a 
meridional  section,  one  sees  the  nerve-bundles  curve  over  the  chorioidal 
foramen  in  bows  into  the  line  of  the  optic-nerve  axis.  Since  the  nerve- 
fiber  layer  and  with  it  the  entire  thickness  of  the  retina  increases  in 
thickness  toward  the  optic  nerve,  this  transition  area  bulges  a  little 
toward  the  interior  of  the  eye  {papilla  nervi  optici).  The  prominence 
is,  however,  insignificant  and  scarcely  deserves  the  name  papilla,  espe- 


THE  OPTIC  NERVE  89 

cially  since  what  one  designates  as  such  in  ophthalmoscopy  has  nothing 
whatever  to  do  with  the  prominence. 

A  depression  arises  in  the  middle  of  the  papilla  from  the  bowing  apart 
of  the  nerve-fibers;  sometimes  this  has  the  form  of  a  funnel,  sometimes 
that  of  a  crater.  The  first  form,  characterized  by  an  ending  in  a  sharp 
angle,  is  called  the  vessel  funnel;  the  latter  form,  showing  a  more  or 
less  fiat  floor,  is  called  the  physiologic  excavation  (Ex).  These  terms  are 
used  with  respect  to  the  variations  in  the  ophthalmoscopic  picture;  an 
essential  anatomic  difference,  aside  from  the  form  of  the  depression,  does 
not  exist.  In  general,  the  form  and  size  of  the  depression  is  subject  to 
very  great  individual  variation. 

The  retinal  portion  of  the  optic  nerve  really  forms  only  a  ring;  the 
temporal  quadrant  of  this  ring  is  lower  and  thinner  than  the  other  three 
quadrants  as  a  result  of  the  lesser  bulk  of  the  nerve-fiber  layer  on  this  side 
of  the  optic  nerve.  The  physiologic  excavation,  therefore,  does  not  lie 
exactly  in  the  middle  of  the  optic  nerve,  but  is  shifted  a  little  to  the  tem- 
poral side.  The  larger  blood-vessels  mount  up  along  the  nasal  wall  of  the 
excavation  to  its  depths;  the  temporal  border  is  crossed  only  by  very 
fine  vessels  going  to  the  fovea. 

The  blood-vessels  of  the  retina  come  together  in  the  upper  and  lower 
quadrant  of  this  ring  and  unite  into  upper  and  lower  main  branches 
(arteriae  or  venae  papiUares  superior  et  inferior).  The  union  of  the  two 
arteriae  papiUares  into  a  trunk  (arteria  centralis  retinae)  takes  place 
while  the  vessels  are  still  in  the  level  of  the  retina  at  an  angle  of  about 
90°;  still  the  plane  of  this  angle  is  usually  placed  sagittally.  The  two 
venae  papiUares,  however,  remain  separated  up  to  the  beginning  of  the 
scleral  portion  of  the  optic  nerve  and  first  unite  here  in  it,  i.e.,  in  the 
lamina  cribrosa  or  on  its  inner  surface.  A  very  considerable  part  of 
the  arteria  centralis  retinae,  therefore,  lies  inside  the  lamina  cribrosa, 
whereas  the  vena  centralis  retinae  is  just  formed  at  it. 

In  the  retinal  part  of  the  optic  nerve  the  vessels  lie  wholly  super- 
ficial, i.e.,  they  are  not  covered  by  nerve-fibers;  on  the  other  hand  they 
possess  a  thin  glial  covering  on  the  vitreous  side.  This  attains  a  special 
thickness  on  the  floor  of  the  physiologic  excavation  and,  to  a  certain 
extent,  forms  the  bulbar  closure  of  the  bundle,  the  central  supporting 
tissue  strand,  which  accompanies  the  arteria  and  vena  centralis  in  their 
further  course  in  the  axis  of  the  optic  nerve.  The  glial  covering  of  the 
floor  of  the  excavation  has  been  designated  as  the  central  supporting 
tissue  meniscus  (Me)  in  this  sense  by  Kuhnt.  Sometimes  this  tissue 
extends  still  farther  into  the  optic  nerve  along  the  central  vessels 
(Elschnig's  intercalary  tissue,  52). 


90  ANATOMY  AND  HISTOLOGY  OF  THE  HUMAN  EYEBALL 

An  actual  liniilans  inlerna,  as  defined  on  pp.  79-80,  does  not  exist  in 
the  region  of  the  physiologic  excavation.  For  this  is  the  place  where  the 
arteria  centralis  retinae  goes  over  into  the  art.  hyaloidea  in  fetal  life,  and 
here  lies  the  entrance  of  the  canalis  hyaloideus  (about  which  contention 
has  again  recently  arisen;  cf.  chap.  xi).  Pathologic  cases  show  that  a 
special  delimitation  from  the  vitreous  fails  here;  for  vascularized  tissue 
very  easily  grows  out  into  the  vitreous  from  this  place  in  inflammations 
{retinitis  proliferans,  cicatrices  in  the  later  stages  of  the  septic  endophal- 
mitis). 

The  rest  of  the  layers  of  the  retina  (from  ganglion-cell  to  rod-and-cone 
layer)  end  at  the  border  of  the  optic  nerve,  the  inner  earlier  than  the 
outer  layers,  corresponding  to  the  bow-form  course  of  the  nerve-fibers. 
Furthermore,  the  layers  thin  out  toward  the  border  and  thereby  compen- 
sate for  the  increase  in  thickness  in  the  nerve-fiber  layer  to  a  certain  extent. 
As  a  matter  of  fact,  however,  these  layers  do  not  reach  clear  up  to  the  optic 
nerve  but  a  thinner  or  thicker  non-stratified  layer  of  tissue,  the  inter- 
mediary tissue  of  Kuhnt  (129)  {im),  is  interposed  between  the  border  of 
the  retina  and  the  most  peripheral  nerve-fiber  bundles. 

The  pigment  epithelium  reaches  up  to  the  intermediary  tissue  at 
the  optic  nerve,  or  nearly  so;  its  border  portions  are,  however,  often  not 
uniformly  developed  and  show  abnormalities  of  pigmentation,  i.e.,  an 
excessive  pigmentation  or  one  defective,  even  to  complete  loss  of  color. 

However,  the  pigment  epithelium  never  extends  as  far  as  does  the 
glass  membrane  of  the  chorioidea.  Of  all  the  layers  of  the  retina  and 
chorioidea,  this  is  the  only  one  which  reaches  clear  up  to  the  optic  nerve; 
indeed,  its  slightly  forward  curved  end  even  covers  it  over  somewhat. 

The  hole  in  the  glass  membrane  of  the  chorioidea  {foramen  opticiim 
laminae  vitreae  chorioideae)  forms  the  inner  opening  of  the  optic-nerve 
canal  (scleral,  sclerotic,  or  entrance  canal  of  the  optic  nerve) ;  it  is  formed 
by  the  union  of  the  foramina  optica  chorioideae  and  sclerae  and  measures 
some  0.5  mm  in  length.  Its  inner  opening  is  about  1.5  mm  wide; 
the  outer  opening,  measured  at  the  level  of  the  outer  surface  of  the 
lamina  cribrosa  or  pial  sheath,  is  usually  much  wider.  Its  cross-section 
varies  only  a  little  or  not  at  all  from  the  form  of  a  circle;  the  longitudinal 
section,  on  the  other  hand,  varies  remarkably  in  individuals.  The  varieties 
will  be  discussed  more  in  detail  later  on. 

The  wall  of  the  canal  is  formed  by  a  white  or  whitish-colored  fibrous 
tissue,  which  is  plainly  set  off  from  the  chorioidal  layers,  but  not  from 
adjacent  sclera  and,  therefore,  appears  as  a  continuation  of  it  like  a 
selvage  surmounting  the  foramen  opticum  sclerae  inwardly  and  extend- 
ing clear  to  the  glass  membrane  of  the  chorioidea  {Gr). 


THE  OPTIC  NERVE  91 

This  tissue  has  been  called  the  connective-tissue  ring,  the  scleral  ring, 
the  sheath  extension,  and  the  border  tissue.  I  prefer  the  latter  (Elschnig's 
term),  because  it  does  not  commit  one  to  anything.  This  border  tissue 
is  much  more  strongly  developed  on  the  temporal  than  on  the  nasal  side. 
It  separates  the  rest  of  the  layers  of  the  chorioidea  (from  the  choriocapil- 
laris  to  the  suprachorioidea)  from  the  optic  nerve,  so  that  none  of  the 
layers  come  in  direct  contact  with  the  optic  nerve.  This  border  tissue 
shows  its  greatest  thickness  at  the  level  of  the  inner  surface  of  the  sclera; 
outside  this  it  thins  rapidly  and  has  completely  disappeared  before  the 
outer  opening  of  the  optic-nerve  canal  is  reached.  It  does  not  go  over 
into  the  pial  sheath  of  the  optic  nerve,  for  this  has  altogether  another 
histologic  composition.  The  name  "sheath  extension"  is  not  at  all  well 
chosen,  therefore. 

The  elements  of  the  border  tissue  do,  however,  probably  extend  into 
the  framework  of  the  optic  nerve,  and  especially  at  the  end  of  the  inner 
surface  of  the  sclera.  It  builds  a  thick  trabeculum  in  connection  with  the 
elements  of  the  sclera  proper  at  the  foramen  opticum  sclerae,  and  this 
surrounds  the  individual  optic-nerve  fiber-bundles  and  separates  them 
from  one  another.  In  and  of  itself  this  framework  has  the  appear- 
ance of  a  sieve;  it,  therefore,  bears  the  name  cribriform  plate  {lamina 
cribrosa,  Lc). 

The  optic-nerve  canal  shelters  the  chorioidal  and  scleral  portions  of  the 
non-medullated  section  of  the  optic  nerve.  The  former  is  a  solid  strand, 
even  when  the  physiologic  excavation  is  not  very  deep.  This  portion  as 
well  as  the  scleral  and  the  adjoining  meduUated  section  has,  of  course,  the 
form  of  a  ring,  if  one  takes  only  the  nerve-fiber  mass  into  consideration; 
the  lumen  of  this  ring  is  wholly  filled  out  by  connective  tissue  and  blood- 
vessels. Commencing  at  least  with  the  scleral  portion  of  the  optic  nerve, 
this  tissue  and  these  vessels  form  a  round  cord,  the  central  supporting 
tissue  strand  {cB),  which  courses  exactly  in  the  axis  of  the  optic  nerve. 
It  contains  one  large  artery  (the  arteria  centralis  retinae)  on  the  nasal  side, 
and,  as  a  rule,  only  one  large  vein  (the  vena  centralis  retinae)  on  the 
temporal  side.  The  lamina  cribrosa  has  a  large  central  opening  for  the 
passage  of  this  strand,  and  is,  moreover,  united  to  it  by  means  of  tissue. 

In  the  entire  non-medullated  section  of  the  optic  nerve  the  individual 
nerve-fiber  bundles  remain  strictly  separated  from  one  another;  there 
are  no  anastomoses.  The  optic-nerve  trabeculum  fills  out  its  interspaces 
and  this,  therefore,  forms  closed  ensheathing  walls  between  the  nerve 
bundles. 

An  exact  longitudinal  section  through  the  non-medullated  part  of  the 
optic  nerve,  which,  indeed,  cannot  always  be  obtained  on  account  of  the 


92  ANATOMY  AND  HISTOLOGY  OF  THE  HUMAN  EYEBALL 

form  of  the  scleral  canal,  shows,  therefore,  a  regular  longitudinal  strialion 
caused  by  the  alternation  of  non-nucleated  nerve-fiber  bundles  and  nuclear- 
rich  separating  walls.  Seen  in  longitudinal  section,  these  are  called  the 
nuclear  columns  (Ks).  The  cross-section,  on  the  other  hand,  shows  the 
ensheathing  walls  as  a  continuous  network  of  rounded  meshes. 

The  framework  is  weakly  developed  in  the  retinal  portion  and  at  the 
level  of  the  inner  opening  of  the  optic-nerve  canal;  it  thickens,  however, 
in  the  chorioidal  portion,  as  the  optic-nerve  canal  widens  and  the  nerve 
bundles  spread  apart.  The  fibrillation  becomes  more  and  more  plainly 
transverse  as  one  approaches  the  lamina  cribrosa;  the  trabeculum  of  the 
chorioidal  portion  is,  therefore,  called  the  lamina  chorioidalis  (Lch)  by 
many,  while  our  lamina  cribrosa  is  given  the  name  lamina  scleralis. 

This  latter  is  nothing  else  than  the  framework  of  the  scleral  portion; 
it  is  differentiated  from  the  remaining  portions  of  the  optic-nerve  frame- 
work histologically,  however,  and  is  less  developed  than  these.  It  does 
not,  therefore,  deserve  to  be  described  as  a  special  structure. 

It  is  useless  to  discuss  whether  the  lamina  cribrosa  is  a  continuation  of  the  inner 
scleral  layers  or  a  modification  of  the  septal  system  of  the  medullated  section  of  the 
optic  nerve.  Each  view  has  as  much  for  as  against  it.  The  same  is  true  of  the  ques- 
tion whether  the  border  tissue  belongs  to  the  chorioidea  or  to  the  sclera  or  is  a  continua- 
tion of  the  pial  sheath. 

The  fact  is  that  here  various  supporting  tissue  structures  meet  and  merge.  The 
supporting  tissue  is  continuous,  as  everywhere  else  in  the  body,  and  we  only  separate 
various  portions  from  one  another  for  purposes  of  description. 

Taken  as  a  whole,  the  lamina  cribrosa  shows  a  certain  concavity 
inward,  more  outspoken  in  the  more  overhanging  nasal  side  of  the  optic- 
nerve  canal.  Its  fibers,  therefore,  show  an  oblique  course  at  the  border, 
i.e.,  they  are  not  parallel  to  the  inner  surface  of  the  chorioidea;  and  when 
one  makes  a  cross-section  of  the  optic  nerve  at  the  level  of  the  lamina 
cribrosa,  the  center  of  the  section  shows  layers  of  the  lamina  cribrosa  lying 
farther  inward  than  do  the  marginal  portions  of  the  lamina  cribrosa. 

The  thickness  of  the  lamina  can  be  placed  at  o .  2  to  o .  3  mm;  it  cannot 
be  given  accurately,  because  the  cribriform  plate  is  not  very  sharply 
demarkated  inwardly  or  outwardly.  In  any  case  the  lamina  cribrosa 
completely  or  nearly  completely  fills  out  the  foramen  opticum  sclerae,  and 
the  border  of  its  outer  surface  lies  pretty  nearly  at  the  level  of  the  root  of 
the  pial  sheath  or  the  bulbar  end  of  the  intervaginal  space  (/).  The 
center  of  the  outer  surface,  however,  lies  considerably  behind  this  level. 

The  fibrillation  of  the  cribriform  plate  is  tense  and  directed  transversely, 
i.e.,  wholly,  or  almost,  at  right  angles  to  the  course  of  the  nerve  bundles; 
its  trabeculum  is  much  thicker  than  the  framework  of  the  chorioidal 


THE  OPTIC  NERVE  93 

portion  of  the  optic  nerve.     For  this  reason  it  is  very  easily  differentiated 
from  the  adjoining  portions  of  the  optic-nerve  framework. 

The  peculiarities  of  its  structure  come  out  still  more  clearly  on  the 
surface-section,  i.e.,  on  cross-section  of  the  nerve  at  the  level  of  the 
lamina  cribrosa  (PL  VI,  i).  The  trabeculae  are  broad,  the  angles  are 
rounded  off,  the  fibers  course  straight,  the  nuclei  are  numerous.  Above 
everything  else,  however,  each  nerve-fiber  bundle  is  strictly  separated 
from  its  neighbor;  no  interruptions  of  the  framework  are  present. 


Histologically,  the  non-medullated  section  of  the  optic  nerve  agrees 
most  closely  with  the  nerve-fiber  layer  of  the  retina.  Its  varicose  non- 
medullated  fibers  are  arranged  in  plainly  separated  bundles;  immeasur- 
ably fine  glial  fibers  support  the  nerve-fibers  and  interlace  with  them. 
Golgi  preparations  show  that  part  of  the  glial  fibers  course  longitudinally 
or  obliquely,  but  the  main  mass  courses  transversely,  i.e.,  perpendicular 
to  the  direction  of  the  nerve-fibers  between  which  they  run.  The  direc- 
tion of  this  portion  of  the  glial  fibrillation,  is  moreover,  to  be  made  out 
without  recourse  to  special  stains,  for  these  fibers  appear  in  part  as  fine 
cross-lines  crossing  the  longitudinal  striation  (brought  out  by  the  nerve- 
fibers),  in  part  as  little  points  (cross-sections)  between  the  nerve-fibers. 

Glial  cells  appear  only  here  and  there  inside  the  nerve-fiber  bundles, 
in  large  numbers,  on  the  other  hand,  between  the  bundles,  so  that  the 
framework  of  the  optic  nerve  seems  rich  in  cells  everywhere.  The  indi- 
vidual glial  cells  have  a  rounded  or  oval  nucleus  of  6  to  9  mu  in  length  and 
an  irregular  stellate  protoplasmic  cell-body.  The  glial  fibers  are  in  close 
relation  to  the  cells  through  whose  protoplasm  they  course.  However, 
as  Weigert's  neuroglia  stain  shows,  they  are  different  from  the  proto- 
plasm and  constitute  independent  fibers  lying  at  one  and  the  same  time 
infra-  and  extracellular,  grouped  about  the  nuclear  center.  The  Golgi 
stain  does  not  differentiate  cells  and  fibers,  and  by  this  method  the  glial 
cells,  therefore,  show  up  as  longish,  deep  black  masses.  A  large  number 
of  extremely  fine  processes  go  out  in  the  most  varied  directions  (so-called 
spider  cells). 

Kuhnt's  intermediary  tissue  consists  of  pure  glial  tissue,  i.e.,  it  con- 
tains no  nerve-fibers;  the  same  is  true  of  the  covering  of  the  physiologic 
excavations  and  the  central  supporting  tissue  meniscus.  The  fibrillation 
is  circular  in  the  intermediary  tissue  and  in  the  lateral  walls  of  the  excava- 
tion; in  the  meniscus  the  fibers  interlace  in  various  directions  (Jacoby, 
109). 

The  gliae  also  appear  as  a  constituent  of  the  border  of  the  optic  nerve. 


94  ANATOMY  AND  HISTOLOGY  OF  THE  HUMAN  EYEBALL 

The  border  tissue  is  more  or  less  richly  permeated  with  glial  fibers  or 
consists  almost  exclusively  of  glia  (Elschnig,  52),  at  least  in  the  portions 
immediately  bordering  on  the  optic  nerve.  In  my  own  experience  this 
latter  does  not,  indeed,  seem  to  be  the  usual  occurrence.  As  a  rule  the 
foundation  of  the  border  tissue  is  a  dense  collagenous  tissue  such  as  is 
found  in  the  sclera;  a  sharp  separation  of  the  two  is  not,  therefore,  pos- 
sible. The  fibrillation  is  mainly  circular,  the  bundles  are  finer  than  in  the 
sclera  proper.  A  rich  mass  of  elastic  fibers  is  intermingled;  near  the  optic 
nerve  and  at  the  level  of  the  inner  surface  of  the  sclera  the  amount  of 
elastic  fibers  attains  its  maximum;  this  place,  therefore,  takes  on  an 
especially  dark  color  after  elective  staining  (orcein,  etc.).  Plnally,  pig- 
ment cells  of  the  chorioidea  (chromatophores)  enter  into  the  composition 
of  the  border  tissue  in  varying  amount.  In  addition,  there  is  still  the 
glial  tissue  shown  by  Jacoby.  It  is  true  that  such  elements  recjuire  a 
special  elective  stain  (Weigert's  glia  stain)  for  their  demonstration,  which 
unfortunately  often  miscarries.  Still  one  can  recognize  the  presence  of 
glial  fibers,  to  a  certain  extent  without  any  such  stain,  although  not  every- 
thing in  the  border  tissue  which  stains  yellow  by  Van  Gieson's  stain  takes 
the  orcein  stain. 

The  lamina  cribrosa  is  similarly  constituted  (PI.  VI,  i);  glial  fibers 
and  cells  with  elongated,  transversely  placed  nuclei,  elastic  fibers,  collage- 
nous fibers  and  numerous  capillaries  make  up  the  cribriform  plate.  The 
latter  arise  from  the  Hallerian  or  vascular  circle  of  Zinn  (see  p.  25), 
along  with  elements  of  the  sclera  (collagenous  and  elastic  fibers),  and  enter 
the  cribriform  plate  in  a  meridional  direction.  The  majority  of  the  struc- 
tural elements  seem,  however,  to  come  out  of  the  border  tissue,  for  surface- 
sections  show  them  bending  oblicjuely  out  of  their  circular  course  toward 
the  axis  of  the  eye. 

The  indistinct  demarkation  of  the  lamina  cribrosa  from  the  neighbor- 
ing portions  of  the  framework  of  the  optic  nerve  has  been  already  reported. 
The  histologic  structure  of  the  framework  of  the  optic  nerve  makes  this 
clear.  As  one  proceeds  inward  (in  the  centrifugal  direction)  one  notes 
that  the  collagenous  and  elastic  elements  become  more  and  more  sparse, 
so  that  finally  there  remains  only  a  fine  glial  tissue  with  plain  transverse 
fibrillations  and  elongated  nuclei ;  the  elastic  fibers  persist  longest  in  the 
marginal  portion  of  the  optic-nerve  framework.  Thus,  one  always  finds 
a  few  fine  elastic  fibers  radiating  out  of  the  border  tissue  into  the  frame- 
work of  the  optic  nerve  at  the  level  of  the  chorioidea,  but  they  are  limited 
to  the  immediate  neighborhood  of  the  border  tissue.  The  framework 
of  the  chorioidal  portion  of  the  optic  nerve  as  a  whole  consists  entirely  of 
glia,  and  only  the  more  outspoken  transverse  course  and  the  greater  size 


THE  OPTIC  NERVE  95 

and  richness  of  the  fibrillation  brings  to  mind  and  seems  to  justify  the 
name  lamina  chorioidalis  in  the  lamina  crihrosa. 

When  one  proceeds  outward  (in  the  centripetal  direction),  on  the 
other  hand,  one  notes  that  the  glia  constantly  decreases  and  the  collage- 
nous fibrilla  increases.  In  this  way  the  cribriform  plate  goes  over  into  the 
septal  system  of  the  medullated  section  of  the  optic  nerve.  This  S3-stem 
contains  only  collagenous  and  elastic  fibrillae. 

Blood-vessels  are  found  in  all  portions  of  the  optic  nerve,  their  mass 
and  origin,  only,  vary ;  but  they  are  found  only  in  the  framework  (between 
the  bundles),  not  inside  the  nerve-fiber  bundles.  The  retinal  and  chori- 
oidal  section  of  the  optic  nerve,  however,  contains  no  mesodermal  ele- 
ments in  its  framework,  aside  from  the  vessels  and  their  adventitia. 
These  first  appear  in  the  lamina  cribrosa,  the  elastic  elements  first  and 
then  the  collagenous  fibrillae.  The  lamina  cribrosa,  therefore,  forms  a 
sort  of  transition  structure  between  the  purely  ectodermal  (glial)  frame- 
work of  the  retinal  and  chorioidal  section  and  the  partly  mesodermal, 
partly  glial  framework  of  the  medullated  section.  The  lamina  cribrosa, 
as  well  as  the  border  tissue,  is,  however,  especially  characterized  by  the 
intricate  interweaving  of  the  ectodermal  and  mesodermal  elements, 
whereas  in  the  medullated  portion  of  the  optic  nerve,  as  well  as  in  the 
central  supporting  tissue  strand  and  its  branches  (the  retinal  vessels),  the 
ectodermal  and  mesodermal  elements  are  sharply  separated  and  plainly 
set  off  from  each  other. 

According  to  Krueckmann  (124),  who  repeatedly  quotes  Held,  this  sharp  separa- 
tion is  due  to  the  fact  that  the  glial  fibers  abut  upon  the  mesodermal  tissue  by  conical 
formed  ends — "end  feet,"  and  a  border  membrane  similar  to  the  limitans  perivascularis, 
described  in  the  nerve-fiber  layer  of  the  retina  (p.  78),  arises,  by  the  confluence  of  these 
little  extremities.  As  a  result  of  this,  according  to  Krueckmann,  the  glial  portion  of 
the  border  tissue  is  separated  from  the  mesodermal  portion,  as  ev-erywhere  else,  by 
limitans  of  this  nature. 

A  splitting  up  of  the  lamina  vilrea  at  the  margin  of  the  optic  nerve,  once  described 
by  Heine  (91)  does  not  exist.  The  membrane  named  ends  abruptly  at  the  optic  nerve; 
it  is,  however,  so  firmly  fastened  to  the  framework  of  the  nerve  that  this  follows  every 
pull  upon  the  lamina  vilrea.  The  tensely  spanned  glial  fibers  of  the  optic-nerve  frame- 
work in  myopic  eyes  then  appear  to  be  derived  from  the  edge  of  the  lamina  vilrea. 

2.       VARIETIES   OF    THE   NON-MEDULLATED    SECTION   OF   THE 
OPTIC   NERVE 

The  variations  found  in  this  region  have  been  thoroughly  studied  by 
Elschnig  (52)  and  accurately  analyzed  in  their  relations  to  the  ophthalmo- 
scopic picture,  as  well.  I  will  here  only  very  briefly  sketch  the  form  of 
the  varieties  found. 

In  as  far  as  they  relate  to  ophthalmoscopy,  one  can  really  look  upon 


96  ANATOMY  AND  HISTOLOGY  OF  THK  HUMAN  EYEBALL 

all  of  them  as  varieties  of  the  oi)tic-ner\fe  canal;  the  formation  of  the 
other  parts  depends  for  the  most  part  upon  the  structure  of  the  optic- 
nerve  canal.  The  nerve-canal  varies  with  respect  to  its  width,  its  form, 
and  its  direction. 

Numerical  estimates  of  the  width  of  the  nerve-canal  must  be  made  as  if 
of  a  certain  level;  the  inner  opening  is  adapted  for  this  purpose,  because 
it  is  well  characterized  and  can  be  sharply  brought  into  view,  on  the  one 
hand,  and  conveys  most  of  interest  in  the  relation  to  the  ophthalmo- 
scopic image  of  the  papilla,  on  the  other  hand.  Its  horizontal  diameter 
varies  between  1.26  and  i .  6  mm  (a  mean  of  1.5)  according  to  my 
measurements.  Its  form  varies  only  a  little  from  that  of  a  circle;  at 
times  the  vertical  diameter  is  a  trifle  greater  and  the  opening,  therefore, 
weakly  elliptical. 

The  form  of  the  nerve-canal  varies  from  that  of  a  cone  to  a  cylinder. 
The  sides  of  this  basic  form  can  be  straight  or  bayed  out  or  in. 

By  the  direction  of  the  nerve-canal  we  understand  its  axis,  character- 
ized anatomically  by  the  central  vessels.  This  direction  may  be  perpen- 
dicular to  the  inner  surface  of  the  chorioidea,  i.e.,  the  nerve-canal  may  form 
a  straight  cone  or  cylinder,  or  be  inclined.  In  the  latter  case  the  center 
of  the  inner  opening  of  the  canal  seems  displaced  away  from  the  outer 
opening  (the  lamina  cribrosa) ;  the  nerve-canal  is  oblique,  and,  as  the  case 
may  be,  temporally  oblique,  nasally  oblique,  or  inferiorly  oblique,  accord- 
ing to  the  direction  in  which  the  inner  opening  is  displaced.  The  central 
vessels,  of  course,  take  part  in  this  oblicjuity. 

The  size  of  the  excavation  depends  principally  upon  the  width  of  the 
optic-nerve  canal ;  in  a  narrow,  cylindrical  canal  a  vessel  funnel  only,  not 
a  genuine  excavation,  is  formed.  The  form  of  the  excavation  is  deter- 
mined by  the  direction  of  the  optic-nerve  canal.  In  a  temporally  oblique 
canal,  the  nasal  border  of  the  excavation  is  steep,  indeed,  even  overhang- 
ing, the  temporal  flat;  in  an  inferiorly  oblicjue  canal  the  upper  border  is 
steep;  in  a  nasally  oblique  the  temporal  border  should  be  steep,  but  this 
difference  is  not  well  enough  marked  to  be  of  moment,  because  the 
temporal  border  is  usually  so  very  low. 

In  the  temporally  oblique  canal  the  border  tissue  is  especially  well 
developed  on  the  temporal  side;  in  a  wide  canal  the  glial  portion  pre- 
dominates, as  if  the  glial  tissue  would  fill  out  the  superfluous  space. 

A  light  temporal  obliquity  of  the  nerve-canal  is  the  most  frequent 
form;  the  nasal  side  then  forms  a  more  acute  angle  with  the  inner  surface 
of  the  chorioidea  than  does  the  temporal  side.  Marked  obliquity  (so 
that  the  angle  named  is  obtuse  on  the  temporal  side)  does,  indeed,  occur 
in  emmetropic  eyes  as  well,  yet  even  then  it  is  associated  with  a  strik- 


THE  OPTIC  NERVE  97 

ingly  long  optic  axis  and  so  represents  a  transition  to  myopia.  Wholly 
symmetrical  nerve-canals  are  in  any  case  more  rare  than  slightly  oblique. 
Properly  speaking,  nasally  and  inferiorly  oblique  canals  are,  indeed, 
malformations,  since  such  eyes  are  usually  below  par  in  function;  yet 
it  is  no  more  permissible  to  draw  a  sharp  distinction  between  normal 
and  malformed  eyes  here  than  it  is  in  myopia. 

3.      THE  SIGNIFICANCE  OF  THE  OPHTHALMOSCOPIC  PICTURE  IN  RELATION 
TO   THE   ANATOMY   OF   THIS   REGION 

That  which  one  calls  the  disc  or  papilla  in  ophthalmoscopy  (PI. 
VII,  i)  corresponds  to  the  area  of  the  lamina  cribrosa  visible  in  the  inner 
opening  of  the  optic-nerve  canal.  In  symmetric  or  only  slightly  oblicjue 
canals  the  margin  of  the  inner  opening  forms  a  border  projecting  (over- 
hanging) on  all  sides.  One  can,  therefore,  never  see  the  side  walls  of  the 
canal,  and  the  inner  opening  often  seems  to  be  entirely  filled  out  by  the 
lamina  cribrosa. 

The  lamina  cribrosa,  in  and  of  itself,  appears  clear  white  with  washed- 
out  gray  stipplings;  the  latter  correspond  to  the  translucent  bundles  of 
optic-nerve  fibers  which  pass  through  the  cribriform  plate.  In  normal 
eyes  the  lamina  cribrosa  never  lies  completely  bared.  The  layer  which 
covers  it  is  only  so  thin  over  the  floor  of  the  physiologic  excavation  that 
the  white  of  the  cribriform  plate  and  also  part  of  the  stippling  is  visible. 
In  the  marginal  portions  of  the  optic-nerve  papilla  there  is  such  a  thick 
layer  of  nerve  tissue  in  front  of  the  lamina  cribrosa  that  the  stippling  is 
invisible  and  a  uniform  reddish  color  results.  The  latter  is  to  be  looked 
upon  as  the  actual  color  of  the  nerve-fiber  mass  and,  on  its  part,  is  due 
again  to  the  blood  contained  in  the  capillaries.  On  the  other  hand,  the 
grayish  admixture  which  one  sees  so  often  in  this  marginal  portion  of  the 
optic  disc  is  solely  a  contrast  in  appearance  in  connection  with  the  clear 
floor  of  the  excavation. 

The  red  of  the  eyeground  and  the  peculiar  granulation  comes  mainly 
from  the  pigment  epithelium.  The  part  played  by  the  blood  of  the  chori- 
oidal  vessels  is  almost  nil  in  a  smoothly  red  eyeground,  according  to 
Marx  (147);  that  is  to  say,  in  a  fundus  of  such  appearance  the  pigment 
epithelium  is  so  densely  pigmented  that  one  cannot  see  the  chorioidal 
vessels  through  it.  When  the  chorioidal  vessels  are  visible,  as  in  less 
pigmented  epithelium,  the  blood  in  the  vessels  as  well  as  the  chorioidal 
stroma  has  an  influence  upon  the  color  of  the  fundus.  The  chorioidal 
vessels  appear  as  red  stripes;  the  interspaces,  when  they  are  densely 
pigmented,  appear  darker,  blacker  (tessellated  fundus),  when  they  are 
weakly  pigmented  lighter,  brownish  to  yellowish.     The  contours  of  the 


98  ANATOMY  AND  HIS'l'OLOGV   OF  THE  HUMAN  EVKHALL 

chorioidal  vessels  are  very  indislincl,  because  the  pigment  epithelium 
obscures  these  contours  like  a  brown  veil. 

Only  when  the  pigment  epithelium  contains  absolutely  no  pigment  at 
all  and  the  chorioidal  stroma  is,  likewise,  non-pigmented  (albinotic 
fundus)  are  the  chorioidal  vessels  visible  as  plainly  contoured  red  stripes 
on  a  yellow-white  background,  for  the  color  of  the  fundus  is  dependent 
upon  chorioidea  alone. 

If  one  excludes  these  cases  which  properly  belong  in  the  territory  of 
anomalies,  one  can  say  that  the  Hmits  of  the  red  color  of  the  fundus  and 
that  of  the  pigment  epithelium  coincide.  In  many  cases  the  red  of  the 
fundus  does  not  reach  clear  up  to  the  optic-nerve  sheaths,  but  a  narrow 
white  strip  (connective  tissue  or  scleral  ring)  is  interposed  between  the 
two.  This  ophthalmoscopic  appearance  may  have  various  anatomic 
bases;  it  may  be  due  either  to  border  tissue  which  is  not  wholly  covered 
by  pigment  epithelium,  or  it  may  be  that  the  side  wall  of  the  scleral  canal, 
which  becomes  of  a  greater  obliquity,  is  visible  in  a  perspective  fore- 
shortening of  the  latter — therefore,  the  lightest  grade  of  distraction  cres- 
cent. The  dark,  black  seam  (chorioidal  ring) ,  by  which  the  red  fundus 
is  often  separated  from  the  optic  disc  or  scleral  ring,  is  due  to  a  heavier 
pigmentation  of  the  epithelium,  therefore  bears  its  name  incorrectly. 

When  the  nerve-canal  is  straight,  the  trunk  of  the  central  artery  is 
not  visible,  or,  properly  speaking,  visible  only  in  optical  cross-section, 
for  the  artery  courses  in  the  line  of  vision  of  the  observer;  for  the  same 
reason,  its  first  branching  makes  an  apparent  angle  of  i8o°. 

It  can  be  closed  in  a  temporally  oblique  nerve-canal,  if  the  nasal 
border  of  the  excavation  is  unusually  steep  or,  indeed,  overhanging,  i.e., 
when  the  reddish-gray  color  of  the  nasal  part  of  the  optic  disc  is  sharply 
set  off  from  the  white  of  the  excavation  and  especially  when  the  first 
division  of  the  arteria  centralis  retinae  forms  a  temporally  open  angle, 
for  in  an  oblique  nerve-canal  the  plane  of  the  branching  is  inclined  away 
from  the  line  of  vision.  The  trunk  of  the  central  artery  is  not,  however, 
visible  as  a  rule,  because  a  very  thick  mass  of  nerve-fibers  covers  up  the 
nasal  part  of  the  optic  disc. 

The  image  is  different  in  a  nasally  oblique  canal.  The  first  of  the 
central  arteries  then  presents  an  angle  open  nasally  (reversed  vessel 
distribution),  and  the  trunk  of  the  central  artery  is  wholly  and  plainly 
visible  over  a  long  stretch,  because  the  nasal  wall  of  the  excavation  on 
which  the  artery  lies  is  now  visible  throughout  almost  its  whole  extent. 
The  vessel  entrance,  i.e.,  the  place  where  the  central  vessels  come  forth 
out  of  the  lamina  cribrosa,  then  appears  strongly  displaced  toward  the 
temporal  side,  for  we  project  everything  into  the  plane  of  the  inner  open- 
ing of  the  nerve-canal,  although  it  really  lies  at  the  deeper  level. 


THE  OPTIC  NERVE  99 

b)    The  Mcdullatcd  Section  of  the  Optic  Nerve 

(PI.  \T,  2) 

This  begins  immediately  behind  the  lamina  cribrosa,  therefore,  about 
o .  5  mm  behind  the  inner  surface  of  the  chorioidea.  Its  thickness  in- 
creases up  to  the  level  of  the  outer  surface  of  the  sclera  and  then  remains 
constant.  Since  the  cross-section  does  not  vary  essentially  from  the  form 
of  a  circle,  the  orbital  section  of  the  optic  nerve  forms  a  cylindrical  strand 
3  to  3  . 5  mm  thick. 

This  strand  is  surrounded  by  connective-tissue  sheaths,  which  are 
united  with  the  sclera  on  one  hand  and  with  the  brain  membranes  on  the 
other,  and  are  named  on  account  of  their  analogy  to  the  latter.  With 
the  addition  of  these  sheaths  the  thickness  of  the  optic  nerve  increases 
to  4  to  4-5  mm. 

Directly  at  the  lamina  cribrosa  a  thin  connective-tissue  layer  is  given 
off  as  an  immediate  covering  for  the  optic  nerve  (the  inner  or  pial  sheath, 
P) ;  it  clings  firmly  to  the  outer  surface  and  extends  into  the  supporting 
tissue  framework  (the  septa).  The  outer  layers  of  the  sclera,  which  are 
not  united  to  the  lamina  cribrosa,  turn  back  with  a  part  of  their  fiber- 
bundles  and  form  a  second  coat  of  some  0.5  mm  thickness  (the  outer 
or  dural  sheath,  D) ;  it  forms  a  hollow  tube  united  to  the  inner  sheath  by 
only  a  few  trabeculae.  Between  these  two  there  lies  a  space,  the  inter- 
vaginal  space;  this  begins  at  the  root  of  the  pial  sheath  and  opens 
centrally  into  the  cavity  of  the  skull. 

The  intervaginal  space  is  divided  into  two  spaces  by  a  delicate  mem- 
brane, the  arachnoidal  sheath  (Ar).  The  outer  space,  between  the  dural 
and  arachnoidal  sheaths,  is  called  the  subdural  space  (sd)  and  is  a 
narrow  cleft-like  space,  broken  only  by  the  trabeculae  which  course  from 
the  dural  to  the  pial  sheath.  The  inner  space,  between  the  arachnoidal 
and  the  pial  sheath,  is  called  the  subarachnoidal  space  (sar)  and  contains 
a  richly  subdivided  system  of  finer  and  grosser  trabeculae  (subarach- 
noidal trabeculae)  constituting  the  union  between  the  arachnoidal  and 
the  pial  sheaths;  this  space  is  wider  than  the  subdural  space,  or  capable 
of  appreciable  widening  in  any  case. 

The  central  supporting  tissue  strand  courses  in  the  axis  of  the  optic 
nerve  for  a  distance  of  7  to  12  mm  from  the  eyeball.  Thereupon  it 
turns  downward  at  almost  a  right  angle  and  leaves  the  optic  nerve.  In 
the  medullary  section  it  has  a  plain  connective-tissue  hull,  which  goes 
over  into  the  pial  sheath  at  the  exit  point  and  so  is  to  be  looked  upon  as  a 
continuation  (invagination)  of  the  pial  sheath.  In  this  way,  too,  the 
orbital  section  of  the  optic  nerve  can  be  further  divided  into  a  (centrally) 
vascular  and  a  (centrally)  avascular  portion. 


loo  ANATOMY  AND  HISTOLOGY  OF  THE  HUMAN  EYEBALL 

I.      THE    SHEATHS    OF    THE    OPTIC   NERVE 

The  outer  and  inner  shealhs  have  exactly  the  same  structure;  they 
consist  of  tough  fibrous  tissue,  in  the  moderately  tortuous  bundles  of 
which  the  collagenous  fibrillae  are  mixed  with  numerous  elastic  fibers. 
These  are  larger  than  are  those  of  the  sclera,  often  branched  and  provided 
with  membranous  expansions  at  the  branching  points.  The  arrangement 
of  the  fibers  on  the  surfaces  turned  toward  the  intervaginal  space  is 
predominantly  a  circular  one,  on  the  surface  turned  away  from  the 
intervaginal  space,  mainly  a  longitudinal  one.  The  cells  are  usually  fiat 
connective-tissue  cells  with  membranous  bodies,  and  have,  therefore,  a 
certain  similarity  to  endothelia ;  they  lie  on  the  surface  of  the  connective- 
tissue  bundle.  So  far  as  one  can  judge  from  ordinary  stains,  there  is  no 
essential  difference  between  them  and  the  cells  of  the  sclera. 

Near  the  bulb  the  dural  sheath  often  splits  into  several  layers  sepa- 
rated by  open  spaces.  I  have  not  been  able  to  convince  myself  that  an 
endothelium  is  present  in  these  spaces;  yet  one  sees  fiat  cells  on  the  walls 
of  the  spaces  here  and  there,  though  these  may  be  only  ordinary  con- 
nective-tissue cells.  The  dural  sheath  shows  an  especially  noticeable 
splitting  where  a  posterior  ciliary  artery  courses  through  it  (see  chap.  xv). 
According  to  authorities,  a  space  (the  supravaginal  space)  lies  outside 
the  dural  sheath;  this  is  a  continuation  of  the  Tenon's  space  and  is 
bordered  outside  by  an  extension  of  Tenon's  fascia.  But  this  supra- 
vaginal space  is  no  more  clearly  a  space  than  is  Tenon's  space,  and  is 
rather  only  a  very  loose  layer  of  connective  tissue  similar  to  subcon- 
junctival tissue,  in  which  fluid  can  easily  broaden  out.  On  the  other 
hand,  the  dural  sheath  is  sharply  delineated  and  provided  with  a  plain 
endotheHum  on  its  inner  surface.  A  good  many  blood-vessels  and  nerves 
are  present,  the  former  principally  on  the  outer  surface  of  the  dural  sheath. 
The  pial  sheath  shows  identically  the  same  structure,  only  it  is  much 
thinner  and  has  a  circular  fibrillation  on  its  outer  surface.  The  trabeculae 
of  union  between  the  dural  and  pial  sheaths  (PI.'  IV,  5,  Vb)  are  pretty  thick 
cylindrical  strands  made  up  of  bundles  of  longitudinally  coursing  collage- 
nous fibrillae  reinforced  by  large  elastic  fibers,  and,  like  all  the  surfaces 
bounding  and  passing  through  the  intervaginal  space,  covered  by  plain 
endothelium.  These  trabeculae  run  through  the  intervaginal  space  very 
obliquely,  so  that  only  cross  or  oblique  cuts  of  these  trabeculae  are  seen 
in  sections;  they  are,  therefore,  easily  distinguished  from  the  elastic  fibers 
and  the  subarachnoidal  trabeculae  by  their  size.  The  vessels,  too,  are 
carried  by  these  trabeculae  to  the  inner  sheath. 

The  rest  of  the  structures  filling  out  the  intervaginal  space  usually 
show  another  make-up;    collagenous  tissue  is,  indeed,  the  substratum 


THE  OPTIC  NERVE  loi 

here  also,  but  elastic  fibers  fail,  and  the  cells  of  the  connective  tissues  are 
replaced  by  endothelium. 

The  actual  arachnoidal  sheath  (Ar)  is  a  continuous  membrane  of 
some  lo  mu  thickness.  The  following  layers  can  be  made  out:  an  endo- 
thelial covering  (outer  endothelium,  aE)  lies  on  the  outside  (that  turned 
toward  the  dural  sheath) ;  its  cells  appear  spindle-form  on  cross-section, 
i.e.,  the  oval  nucleus  is  surrounded  by  some  protoplasm.  One  very 
frequently  sees  proliferations  of  this  endothelium,  even  in  eyes  which  are 
otherwise  normal;  it  then  appears  to  have  several  layers  for  stretches; 
indeed,  even  spherical  pearls,  made  up  of  rounded,  concentrically  strati- 
fied endothelium,  may  form,  and  then  concentric  concrements  develop 
from  a  degenerative  process. 

The  outer  endothelium  is  succeeded  by  a  very  delicate  layer  of  non- 
nucleated  connective  tissue  made  up  of  small  stellate  expansions  whose 
processes  build  a  network  (PL  IV,  6,  a).  Some  clearly  demonstrable 
tiny  bundles  with  a  tortuous  course  go  inward  from  the  centers  of  these 
little  stars.  Several  of  these  little  bundles,  which  consist  solely  of 
collagenous  fibrillae,  course  farther  on  parallel  to  one  another,  and  these 
groups  cross  and  interweave;  in  this  way  there  arises  a  second,  heavier 
layer  of  connective  tissue  (i)  inside  the  stellate  expansions,  made  up  of 
bundles. 

The  inner  endothelium  follows  this  layer;  this  seems  to  be  exactly  like 
the  outer  endothelium,  yet  it  has  no  tendency  to  proliferation.  The 
inner  endothelium  forms  the  inner  surface  of  the  arachnoidal  sheath 
proper,  i.e.,  the  one  turned  toward  the  pial  sheath  (PI.  IV,  5,  iE). 

The  subarachnoidal  trabeculae  (PI.  IV,  5,  sb)  forms  out  of  the 
second  (inner)  connective  tissue;  groups  of  little  bundles  of  fibrillae  unite 
into  a  larger  bundle  and  leave  the  arachnoidal  sheath,  covered  with  a 
continuation  of  the  inner  endothelium.  These  primitive  subarachnoidal 
trabeculae,  therefore,  consist  only  of  non-nucleated  strands  of  collagenous 
fibrillae  covered  by  a  plain  endothelial  membrane  made  up  of  a  thin  layer 
of  protoplasm  strewn  with  oval,  somewhat  prominent  nuclei. 

The  primitive  trabeculae  unite  into  a  meshwork  and  in  this  way 
permeate  the  whole  subarachnoidal  space.  The  nearer  one  approaches 
the  pial  sheath,  the  larger  the  trabeculae  become  (SB),  and  within  these 
larger  trabeculae  are  a  few  cells  and  elastic  fibers  as  well. 

Finally,  these  trabeculae  go  over  into  the  pial  sheath;  their  endo- 
thelium passes  into  the  (outer)  endothelial  covering  of  the  pial  sheath, 
and  their  fibers  into  the  outer  circular  fiber  layer  of  the  connective  tissue. 

The  trabeculae  of  union,  which  traverse  the  entire  intervaginal  space, 
do  not  enter  the  arachnoidal  sheath,  but  the  latter  is  invaginated  inward 


I02  ANATOMY  AND  IIISTOLOGN'  OF  THE  HUMAN  EYEBALL 

along  these  trabeculae  and  envelops  them  still  for  a  long  stretch.  The 
cross-section  of  such  a  trabeculae  of  union  (Vb),  therefore,  shows  a  core 
of  firm  connective  tissue  with  elastic  fibers  surrounded  by  a  delicate  layer 
of  exactly  the  same  structure  as  the  arachnoidal  sheath. 

In  general,  the  subdural  and  subarachnoidal  spaces  are  entirely  sepa- 
rate from  one  another,  for  the  arachnoidal  sheath  is  continuous  and  free 
from  dehiscences. 

The  following  is  to  be  said  concerning  the  union  of  the  sheaths  of  the 
optic  nerve  with  the  sclera.  When  it  is  stated  that  the  outer  half  or 
two-thirds  of  the  sclera  goes  over  into  the  dural  sheath,  it  is  meant  that  the 
intervaginal  space  reaches  to  this  depth.  Only  a  part  of  the  scleral 
fiber-bundles  go  over  into  the  dural  sheath;  these  bundles  in  the  sclera 
course  directly  up  to  the  optic  nerve  and  then  bend  about  in  a  sharp 
bow  into  the  longitudinal  fiber  course  of  the  dural  sheath;  they  are,  there- 
fore, visible  throughout  their  whole  course.  Numerous  cross-sections  of 
scleral  fiber-bundles  also  appear  at  the  insertion  of  the  dural  sheath ;  these 
are  bundles  which  do  not  enter  the  dural  sheath  and  are  deflected  in  flat 
curves  out  around  the  optic  nerve. 

These  deflected  bundles  may  intermingle  extensively  with  these 
which  go  over  into  the  dural  sheath;  the  root  of  the  dural  sheath  is  then 
broad  and  deep.  In  other  eyes  these  deflected  bundles  are  systematically 
compressed  into  a  layer,  and  then  the  dural  sheath  is  sharply  set  oflf 
from  the  sclera.  It  can  come  about,  for  example,  that  the  outer  third  of 
the  sclera  does  not  go  over  into  the  dural  sheath  at  all,  and  thus  appears 
to  be  a  continuation  of  the  middle  third  (consult  Elschnig,  52). 

The  distance  between  the  root  of  the  dural  sheath  and  the  optic  nerve 
is  also  subject  to  variation.  The  greater  this  distance,  the  wider  the 
anterior  end  of  the  intervaginal  space  seems.  When,  therefore,  the  end 
of  the  dural  sheath  retracts  after  enucleation,  as  it  does  regularly  to  a 
greater  or  less  extent,  it  sinks  into  the  widened  end  of  the  intervaginal 
space,  and  this  then  shows  an  angular  bending  about  into  the  course  of 
the  scleral  fibers. 

The  intervaginal  space  is  always  wider  at  its  anterior  end  than  in  the 
orbital  part  of  the  optic  nerve,  because  just  behind  the  lamina  cribrosa 
the  optic  nerve  does  not  yet  possess  its  full  thickness.  The  inner  scleral 
layers,  united  only  with  the  pial  sheath  and  the  lamina  cribrosa,  always 
close  off  the  intervaginal  space  in  front,  and  to  a  certain  extent  form  the 
anterior  wall.  The  form  of  this  anterior  end  of  the  intervaginal  space  is 
subject  to  many  variations  and  often  is  not  alike  on  the  two  sides  of  the 
optic  nerve,  yet  one  cannot  establish  fixed  t>'pes.  The  oblique  direction 
of  the  optic-nerve  canal  is,  as  a  rule,  associated  with  a  more  marked 


THE  OPTIC  NERVE  103 

widening  of  the  intervaginal  space ;  one  finds  such  a  widening  on  the  nasal 
side  in  a  temporally  oblique  canal,  especially. 

The  arachnoidal  sheath  usually  ends  in  the  angle  which  the  dural 
sheath  makes  with  the  anterior  wall  of  the  intervaginal  space,  at  times 
even  farther  back,  for  it  goes  over  into  the  dural  sheath. 

The  pial  sheath  usually  shows  a  thickening  at  its  scleral  end,  so  that 
the  angle  between  it  and  the  anterior  wall  of  the  intervaginal  space  is 
rounded  out.  The  most  anterior  part  of  the  pial  sheath  consists  of  con- 
nective-tissue bundles,  which,  in  the  main,  have  a  circular  course;  since, 
too,  the  neighboring  portions  of  the  sclera  are  almost  exclusively  made  up 
of  such  fiber-bundles,  a  delimitation  of  the  pial  sheath  from  the  sclera  is 
not  possible.  The  longitudinal  fibers  can  be  followed  some  distance 
farther  forward,  but  they  do  not  form  so  compact  a  mass  that  one  can 
speak  of  a  continuation  of  the  pial  sheath  up  to  the  lamina  vitrea  chori- 
oideae.  When,  therefore,  some  authors  speak  of  the  border  tissue  as  a 
sheath  extension,  they  have  chosen  a  subjective  conception,  or  a  form  of 
presentation  and  description,  in  order  to  make  the  complicated  anatomic 
relations  more  demonstrable;  it  is  not  an  extension  in  the  strict  sense  of 
the  word. 

2.       THE    OPTIC-NERVE    TRUNK 

This  consists  of  a  large  number  of  rounded  nerve-fiber  bundles  sepa- 
rated from  one  another  partly  by  glial  tissue  and  partly  by  connective 
tissue  trabeculae.  This  separation  is,  however,  an  incomplete  one.  The 
individual  nerve-fiber  bundles  exchange  fibers  here  and  there. 

The  connective  tissue  trabeculae  (septa)  carry  the  vessels  to  the 
nerve-tissue  and  form  a  closed  system  (septal  system)  united  on  one 
side  with  the  pial  sheath  and  on  the  other  side  with  the  central  con- 
nective-tissue strand,  in  so  far  as  this  is  present;  as  described  above,  it 
goes  over  into  the  lamina  cribrosa  in  front.  The  septa  do  not,  however, 
inclose  the  individual  nerve-fiber  bundles  on  all  sides,  but  only  unite 
groups  of  bundles,  and,  indeed,  they  do  not  form  a  continuous  septal  wall 
between  such  bundles,  but  only  along  stretches;  otherwise  the  framework 
is  made  up  of  oblique  and  cross  trabeculae.  The  grouping  of  the  bundles 
into  septa  changes  in  every  cross-section;  the  partitioning  walls  formed 
by  the  glial  tissue  mount  up  over  the  septa  and  to  a  certain  extent  form 
its  continuation  and  bridge  over  its  interspaces. 

The  make-up  of  the  septal  system  can  most  easily  be  studied  in  longi- 
tudinal sections  after  staining  by  Van  Gieson  (PI.  VI,  4).  The  longitudi- 
nal section  of  the  optic  nerve  gives  a  surface  view  of  the  septa  in  some 
places,  a  longitudinal  section  of  the  septa  in  others. 


I04  ANATOMY  AND  HISTOLOGY  OF  THE  HUMAN  EYEBALL 

In  the  surface  view,  the  septa  (5,)  sometimes  appear  as  expanded 
l)latcs,  sometimes  as  narrow  bundles  of  connective  tissue  carrying  blood- 
vessels of  varying  caliber.  They  bridge  over  a  few  of  the  nerve-fiber 
bundles,  then  bend  out  of  the  plane  of  the  section,  so  that  one  cannot 
follow  them  any  farther.  On  the  longitudinal  section  each  septum  (S^) 
appears  as  a  narrow  strip  of  connective  tissue,  or  as  a  row  of  cross- 
sections  of  rounded  connective-tissue  bundles  broken  by  larger  inter- 
spaces. The  interspaces  are  filled  out  by  rows  of  glial  cells  (G/).  After 
a  short  distance  the  cross-section  of  connective  tissue  stops  altogether 
and  only  the  rows  of  glial  cells  continue;  cross-sections  of  connective 
tissue  appear  thereafter  in  one  or  another  place.  The  glial  cells,  there- 
fore, appear  arranged  in  regular  longitudinal  stripes,  and  when  one 
follows  such  a  stripe  he  sooner  or  later  comes  upon  a  septum. 

In  this  description  I  have  an  ideal  longitudinal  section  in  mind;  it 
is  to  be  remembered  in  the  study  of  the  preparation,  however,  that  even 
after  a  careful  choice  of  the  direction  of  the  section,  it  is  only  rarely  possible 
to  maintain  the  exact  longitudinal  direction  of  the  septa  for  long  stretches, 
because  the  nerve  normally  shows  an  S-shaped  curve,  and  the  end  farthest 
away  from  the  bulb  is  slightly  bent.  The  accurate  production  of  cross- 
sections  is  easier. 

In  such  a  section  (PL  VI,  2)  the  septal  system  appears  more  con- 
tinuous and  better  delimited  and  subdivided  into  irregular  polygonal 
fields  with  rounded  angles.  Yet  even  these  are  not  set  off  on  all  sides, 
and  many  septa  appear  to  be  discontinued  after  they  have  pressed  some 
distance  into  the  nerve-fiber  mass.  The  largest  septa  are  called  the 
primary,  the  weaker  and  more  incomplete  ones  the  secondary  septa. 
Their  incompleteness  is,  however,  only  apparent:  they  are  trabeculae 
which  course  obliquely  to  the  plane  of  section,  and,  therefore,  fall  into  the 
plane  of  section  only  in  part. 

Upon  cross-section,  as  well,  the  "incomplete"  septa  are  continued  by 
empty  glial  tissue,  that  is,  glial  tissue  containing  no  nerve-fibers.  When 
one  stains  the  cross-section  of  the  nerve  by  Weigert's  method  (PI.  VI,  6), 
the  nerve-fiber  bundles  appear  much  sharper  and  more  completely  sepa- 
rated from  one  another,  because  after  this  staining  the  connective  tissue 
and  the  glia  take  on  the  same  light-brown  nuance. 

Aside  from  these  glial  continuations  of  the  "incomplete"  septa, 
glial  cells  are  found  in  the  nerve-fiber  mass,  apparently  in  irregular 
arrangement.  But  this  lack  of  regularity  is  only  an  apparent  one,  for  in 
longitudinal  section  these  glial  cells  correspond  to  regular  rows  which 
accompany  septal  trabeculae,  either  farther  above  or  below. 

From  this  one  comes  to  the  conviction  that  the  glial  cells  in  the 


THE  OPTIC  NERVE  105 

meduUated  section  of  the  nerve,  as  in  the  non-medullated,  are  found 
only  on  the  surface  of  the  individual  nerve-fiber  bundles,  i.e.,  where  these 
are  not  separated  by  septa.  The  pure  glial  septal  walls  arise  as  exten- 
sions of  the  septa.  But  neither  does  the  glial  framework  effect  a 
complete  separation  of  the  individual  nerve-fiber  bundles  from  each 
other,  for  these  fibers  anastomose  in  places,  and  the  cells  lying  inside 
the  nerve-fiber  mass  are  nothing  else  than  the  ends  of  the  glial  frame- 
work at  the  anastomosis  of  the  nerve-fiber  bundles. 

The  tissue  of  the  septa  is  ordinary  connective  tissue;  the  collagenous 
fibrillae  form  delicate  bundles,  which  as  a  rule  course  crosswise  or 
obliquely,  depending  upon  the  direction  of  the  particular  trabecula; 
however,  they  also  course  longitudinally  in  the  case  of  the  larger  septa. 
Elastic  fibers  are  numerous  and,  in  general,  take  the  course  of  the  col- 
lagenous fibrillae.  These  sparse  nuclei  are  slender  and  stain  deeply. 
The  blood-vessels  (PI.  VI,  3,  g)  lie  throughout  in  the  septal  tissue  and, 
with  few  exceptions,  are  branches  of  the  sheath  vessels;  the  heaviest 
part  of  the  septal  system  likewise  carries  the  largest  vessels. 

The  glial  tissue  agrees  wholly  with  that  of  the  non-medullated  section 
of  the  nerve  (see  p.  loi).  As  above  reported,  the  cells  lie  at  the  periphery 
of  the  individual  bundles,  in  part,  therefore,  between  the  septa  and  the 
nerve-fiber  mass.  For  the  most  part,  however,  they  lie  in  the  prolongations 
of  the  septa,  which,  accordingly,  consist  of  cells  and  a  reticulum  of  glial 
fibers.  The  glial  fibers  also  press  into  the  interior  of  the  nerve-fiber 
bundles,  and  insinuate  themselves  between  its  fibers  in  a  cross,  oblique, 
or,  in  part,  longitudinal  direction.  They  are  thickest  on  the  surface 
of  the  nerve-fiber  bundle.  Inward  they  are  less  numerous,  yet  they  per- 
meate all  the  nerve-fiber  bundles,  so  that  one  sees  portions  of  glial  fibers 
(PI.  VI,  5,  gl)  everywhere  between  the  cross-sections  of  the  nerve-fibers. 

The  ghal  tissue  is  sharply  separated  from  the  connective  tissue  of  the 
septa;  neither  connective-tissue  fibers  nor  vessels  press  down  into  the 
nerve-fiber  bundles,  nor  do  glial  fibers  press  into  the  septal  system. 
The  nerve-fiber  mass  often  shrinks  a  little  in  the  hardening  fluid,  and  then 
cleft-like  spaces  form  between  the  septa  and  the  surface  of  the  nerve- 
fiber  mass;  the  spaces  are  mostly  bridged  over  by  glial  fibers,  but  often 
appear  quite  empty.  These  spaces  do  not  possess  an  endothelial  lining; 
they  can,  therefore,  best  be  explained  as  artefacts.  They  are  apparently 
the  same  spaces  as  those  produced  by  injections  of  the  nerve  trunk  and 
which  have  been  looked  upon  as  lymph  spaces  (PI.  VI,  3,  Ly). 

The  nerve-fibers  are  of  the  same  sort  as  those  found  in  the  white  sub- 
stance of  the  brain  and  spinal  cord.  They  are  fine  fibers  consisting  only 
of  an  axis  cylinder  and  medullary  sheath  without  a  sheath  of  Schwann ; 


io6  ANATOMY  AND  HISTOLOGY  OF  THE  HUMAN  EYEBALL 

their  thickness  varies  usually  between  2  and  5  mu  (PI.  VI,  5,  »).  The 
finer  fibers  have  been  looked  upon  as  the  actual  visual  fibers,  the 
thicker  ones  as  pupillary  fibers  (von  Gudden,  Westphal,  and  others). 
When  hardened,  these  fibers  show  varicosities,  i.e.,  nodular  swellings. 
The  very  great  variation  in  the  size  of  the  cross-section  of  the  fibers  is 
due  to  the  fact  that  the  section  cuts  many  fibers  through  the  nodes  and 
others  through  non-nodular  portions. 

The  varicosities  have  been  pretty  generally  looked  upon  as  artificial  products. 
Bartels  (17)  has  demonstrated  a  very  large  number  of  primitive  fibrillae  in  the  axis 
cylinder. 

At  the  periphery  of  the  optic  nerve  a  more  or  less  well-marked,  plain 
layer  of  flattened  and  compressed  bundles  is  found;  this  consists  only  of 
glial  tissue  (fibers  and  cells),  and  contains  no  nerve-fibers  (Greeff's  periph- 
eral glial  mantle,  74)  (PI.  VI,  2,  6,  glm).  A  structure  analogous  to 
this  is  found  in  the  brain  and  spinal  cord;  it  is  indicated  even  in  the 
newborn,  but  is  not  as  plainly  visible  as  later  on  account  of  the  defect- 
ive development  of  the  medullary  sheaths  (Kiribuchi,  118;  Greeff,  74). 
These  bundles  are  separated  from  the  nerve-fiber  bundles  by  stretches 
of  connective  tissue  septa,  coursing  parallel  to  the  pial  sheath  (peripheral 
septa  of  Fuchs,  66). 

At  the  posterior  end  of  the  central  connective-tissue  strand  the 
peripheral  glial  mantle  is  turned  in,  like  the  pial  sheath  itself,  and  continues 
along  the  central  connective-tissue  strand  for  a  varying  distance  (PI. 
VI,  2). 

This  place,  usually  spoken  of  as  the  entrance  of  the  central  vessels, 
lies  below  and  somewhat  nasal,  according  to  Deyl  (38),  almost  straight 
below,  according  to  Strahl  (214).  Its  significance  lies  mainly  in  the  fact 
that  it  corresponds  to  the  posterior  end  of  the  fetal  cleft  (cf.  chap,  xvi), 
and  in  the  developed  eye  represents  the  only  visible  trace  of  this  phase  of 
the  embryologic  development  so  important  for  the  study  of  anomalies. 

The  main  axis  of  the  central  connective-tissue  strand  consists  of  a 
tubular  continuation  of  the  pial  sheath  (for  a  covering),  and  of  the  two 
central  vessels  {arteria  et  vena  centralis  retinae)  as  contents.  The  covering 
consists  of  longitudinally  fibrillated  connective  tissue  and  agrees  in  every 
histologic  particular  with  the  septal  system,  into  which  it  is  directly  con- 
tinued. Between  it  and  the  central  vessels,  and,  likewise,  in  the  inter- 
spaces between  the  two  vessels,  there  is  a  loose  connective  tissue  much 
split  up  into  longitudinal  spaces  (lined  by  epithelium  ?). 

The  central  vessels  give  off  only  a  few  small  branches  in  the  trunk  of 
the  optic  nerve,  and  these  are  mostly  veins.  Therefore,  they  maintain 
their  caliber  unchanged  up  to  the  lamina  cribrosa.     The  arteria  centralis 


THE  OPTIC  XERVE  107 

retinae  does  not,  as  a  rule,  show  any  evidence  of  contraction,  but  has  a 
wide-open  lumen  of  some  o.  13  mm  diameter,  a  weakly  developed  intima, 
a  0.02  mm  thick  muscularis,  and  an  equally  w^eak  adventitia.  The 
vein  wall  consists  solely  of  endothelial  and  connective  tissue. 

Fine  nerve  branches  enter  with  the  central  vessels,  and  form  a 
ganglion-cell-free  plexus  about  the  central  artery;  this  can  be  followed 
into  the  papilla  (Krause,  122). 

It  is  not  rare  to  find  the  optic  nerve  changed  in  a  peculiar  way  on  cross-section; 
one  finds  swollen  or  knotted  areas  in  which  the  nerve-fibers  are  not  cut  across,  as  usual, 
but  longitudinally  or  obliquely;  therefore  they  appear  very  indistinct,  so  that  the 
whole  node,  which,  in  general,  is  sharply  set  off  from  the  normal  tissue,  is  recognized 
even  by  low  magnification  and,  indeed,  macroscopically  in  uncut  tissues  by  another 
color  (PI.  VI,  6,  c).  These  changes  were  described  by  Siegrist  (207),  and  were  con- 
sidered to  be  areas  of  fatty  degeneration.  However,  it  came  out  in  the  discussion  of 
Siegrist's  paper  that  these  appearances  are  well  known,  and  have  usually  been  looked 
upon  as  cadaverous  appearances.  Elschnig  (53)  finally  demonstrated  that  the  cause 
of  these  changes  is  the  bruising  of  the  optic  nerve  by  instruments  in  the  preparation 
of  the  orbital  contents.  The  fleck-form  degeneration  of  the  optic  nerve  of  Siegrist 
is,  according  to  this,  listed  as  an  artefact. 


CHAPTER  IX.     THE  CILIARY  BODY  (CORPUS  CILIARE) 

(PI.  I) 

The  ciliary  body  forms  a  girdle  of  about  5  to  6  mm  in  breadth, 
narrower  on  the  nasal  side  and  above  (4.6  to  5.2  mm) ,  broader  on  the 
temporal  side  and  below  (5.6  to  6.3  mm).  A  meridian  going  obUquely 
from  temporal  and  above,  nasal  and  downward,  separates  the  narrower 
from  the  broader  part  (PL  II,  i).  The  description  of  the  outer  surface 
has  been  given  above  (pp.  12-13),  likewise  the  characteristics  of  the  two 
zones  on  the  inner  surface :  the  orbiculus  ciliaris  and  the  corona  ciliaris 
(p.  10). 

The  orbiculus  ciliaris  is  the  broader  of  the  two  zones.  Some  2  mm 
throughout  belongs  to  the  corona  ciliaris,  the  rest  to  the  orbiculus  ciliaris. 
The  significant  difference  in  the  breadth  of  the  entire  body  depends, 
therefore,  upon  the  expanse  of  the  latter.  In  general,  the  inner  surface 
of  the  orbiculus  ciliaris  is  considerably  darker  than  that  of  the  chorioidea. 
This  depends,  however,  only  upon  the  pigment  epithelium.  The  following 
details  of  its  color  can  be  recognized  under  certain  circumstances. 

Immediately  in  front  of  the  era  serrata  its  color  is  often  darker  than 
in  the  middle,  or  about  a  millimeter  in  front  of  the  ora  serrata  one  sees  an 
especially  dark  girdle, which  reproduces  the  zig-zag  form  of  the  ora  serrata 
with  absolute  accuracy,  although  in  a  meridionally  enlarged  proportion, 


io8  ANATOMY  AND  HISTOLOGY  OF  THE  HUMAN  EYEBALL 

therefore  exaggerated  to  a  certain  extent.  Corresponding  to  the  teeth 
of  the  ora  serrata,  there  are  narrow  radial  striae  which  open  into  the 
corresponding  valleys  of  the  ciliary  body  (striae  ciliares,  O.  Schultze). 

The  dark  girdle  is  not  always  visible;  often  it  is  developed  only  in 
the  broader  part  of  the  orbiculus  ciliaris  and  is  entirely  absent  in  many 
eyes.  But  the  striae  ciliares  (PI.  IV,  lo,  St)  are  always  to  be  seen  except 
in  those  cases  in  which  the  pigmentation  is,  in  general,  so  intense  that  one 
cannot  make  out  any  difference.  The  color  is  usually  darker  in  front 
toward  the  corona  ciliaris. 

Although  one  speaks  of  the  orbiculus  ciliaris  as  the  flat  portion  of  the 
ciliary  body,  this  is  not  to  be  taken  absolutely  literally.  For  example, 
the  dark  girdle  always  shows  a  slight  prominence  when  well  developed. 
Slight  differences  in  level  are  also  to  be  found  over  the  orbiculus.  The 
same  is  true  of  the  striae  ciliares.  In  the  most  anterior  portion  of  the 
orbiculus,  just  behind  the  corona,  one  sees  a  system  of  little  warts  or  folds 
in  many  eyes;  these  are  very  much  smaller  than  the  ciliary  processes  and 
only  visible  when  looked  at  with  the  loupe  under  strong  focal  light  (sun- 
light is  best).  These  warts  (PL  IV,  lo,  w)  are  elongated,  sausage-like 
structures  with  their  long  diameters  meridional;  they  are  often  arranged 
in  chains  and  three  or  four  such  rows  are  found  in  a  ciliary  valley. 

The  corona  ciliaris  is  much  more  uniformly  developed  throughout 
its  entire  circumference  than  in  the  orbiculus  ciliaris.  The  difference 
between  the  nasal  and  temporal  sides  is  slight  and  amounts  to  only  a  few 
tenths  of  a  millimeter  at  the  most. 

The  meridional  white  striation,  so  striking  even  on  macroscopic 
examination,  is  due  to  the  summits  of  the  ciliary  processes  (processus 
ciliares).  These  striae  give  the  name  to  the  zone  and  number  about  70 
in  the  entire  circumference. 

Each  process  (PL  I)  presents  a  plate  or  ridge  projecting  axial  (toward 
the  lens)  and  inward ;  it  is  about  2  mm  long  (in  the  meridional  direction) 
and  o .  8  mm  high  (in  a  radial  direction).  The  free  border,  or  ridge,  of  the 
process — that  turned  toward  the  lens  and  the  vitreous — is  less  pigmented 
than  the  side  surface  and  the  interspace  and,  therefore,  appears  clear  upon 
the  neighboring  dark  background  (PL  IV,  10,  Pc). 

The  interspaces  (ciliary  valleys)  between  the  processes  carry  numerous 
similar  projections;  posteriorly  these  go  over  into  the  above-mentioned 
warts;  in  front  they  become  larger  and  here  and  there  they  grow  to  espe- 
cially large  prominences  {plicae  ciliares)  in  the  neighborhood  of  the  iris 
root,  but  they  always  remain  much  smaller  than  the  ciliary  processes. 
These  plicae  are  as  darkly  pigmented  as  the  floor  of  the  ciliary  valley  and, 
therefore,  are  not  plainly  visible  in  macroscopic  examination.     Finally, 


THE  CILIARY  BODY  109 

the  whole  system  of  elevations  and  projections  is  succeeded  in  front  by 
a  circular  ridge  which  juts  forth  about  opposite  the  border  of  the  lens 
(sims  of  H.  Virchow,  232). 

Smaller  irregularities  in  the  development  of  the  corona  ciliaris  fre- 
quently appear  without  particular  reference  to  location;  thus  here  and 
there  the  ciliary  valleys  are  wider  or  the  individual  processes  are  notably 
lower  than  their  neighbors  (PL  II,  i).  The  marked  individual  variations 
in  this  region  have  recently  been  elucidated  through  the  excellent  draw- 
ings by  Hess  (loi). 

At  its  posterior  border,  the  ciliary  body  is  not  any  thicker  than  the 
peripheral  parts  of  the  chorioidea;  where  the  ciliary  muscle  begins,  how- 
ever, some  3  mm  behind  the  anterior  border,  the  thickness  of  the  ciliary 
body  gradually  increases  and  attains  a  maximum  of  o .  8  mm  at  its  very 
anterior  border.  With  this  maximal  thickness  the  ciliary  body  ceases  as 
such,  as  a  rule,  and  thereby  acquires  a  three-sided  prismatic  form;  an 
outer  surface  is  turned  toward  the  sclera,  an  inner  toward  the  vitreous, 
and  a  narrow  anterior  surface  is  turned  toward  the  center  of  the  cornea 
or  the  pupU. 

The  ledge  formed  by  the  outer  and  anterior  surfaces  borders  on  the 
scleral  roll  (PL  III,  i,  Sw);  here  one  finds  the  insertion  of  the  ciliary 
body  into  the  sclera,  as  well  as  the  anterior  insertion  of  the  uveal  tract, 
in  general.  The  inner  and  the  anterior  surfaces  unite  in  a  rounded  ridge 
projecting  in  the  direction  of  the  border  of  the  lens;  this  will  be  spoken  of 
as  the  inner  ledge ;  it  is  crowned  by  the  sims. 

The  insertion  of  the  iris  root  into  the  anterior  surface  lies  in  the 
neighborhood  of  this  ledge,  whereas  the  meshwork  of  the  iris  angle,  in  so 
far  as  it  does  not  go  over  into  the  scleral  roll,  unites  with  the  peripheral 
parts  of  the  anterior  surface  of  the  ciliary  body.  There  remains  a  narrow 
strip  of  the  anterior  surface  between  the  two  in  many  eyes;  this  is  covered 
only  by  the  innermost  lamellae  of  the  trabeculum  (the  uveal  meshwork) 
and  so  takes  part  in  the  limitation  of  the  anterior  chamber.  In  other 
cases  the  anterior  layers  of  the  iris  extend  to  the  scleral  meshwork  as  a 
much-broken  layer  and  so  cover  over  this  remnant  of  the  anterior  surface. 

The  direction  of  the  anterior  surface  varies  even  under  normal 
conditions,  and  even  more  so  when  one  takes  into  account  eyes  with  an 
abnormally  long  axis;  and,  indeed,  as  follows: 

I.  The  direction  of  the  anterior  surface  is  more  nearly  sagittal;  if 
one  erect  a  line  perpendicular  to  the  inner  surface  of  the  sclera  at  the 
inner  ledge,  the  foot  of  this  line  falls  behind  the  scleral  roll;  the  ciliary 
muscle  is,  in  general,  longer — the  myopic  type,  so  called  because  it  is 
found  in  pronounced  form  in  eyes  with  axial  myopia. 


no  ANATOMY  AND  HISTOLOGY  OF  THE  HUMAN  EYEBALL 

2.  The  anterior  surface  is  more  frontally  placed;  the  perpendicular 
line  drawn  from  the  inner  ledge  to  the  inner  surface  of  the  tunica  fibrosa 
falls  in  front  of  the  insertion  of  the  ciliary  muscle;  the  muscle  is,  in 
general,  shorter— hypermetropic  type,  because  it  is  found  mainly  in  eyes 
of  lessened  axial  length. 

Aside  from  these  individual  differences  one  notes  differences  as  well 
between  the  nasal  and  temporal  sides  in  many  eyes,  such  as  the  ciliary 
body  approaching  the  type  of  hypermetropia  on  the  nasal  side,  and  that 
of  myopia  on  the  temporal  side. 

The  greater  part  of  each  ciliary  process  sets  upon  the  inner  surface,  a 
smaller  part  extends  over  the  inner  ledge  to  the  iris  root,  and  even  reaches 
a  short  way  over  onto  the  back  surface  of  the  iris.  In  a  side  view  the 
ciliary  process  appears  uniformly  rounder,  aside  from  small  indentations, 
similar  to  the  circumference  of  the  pinna  of  the  ear.  But  one  only  obtains 
this  view  in  the  meridionally  bisected  eyeball  or  in  very  thick  meridional 
sections.  The  microscopic  preparation  also  shows  interruptions  and 
defects  in  the  ciliary  processes,  even  when  the  direction  of  section  is  most 
carefully  thought  of  (PI.  I);  these  are  not  actual  interruptions  but  only 
uncapped  depressions  of  the  side  surfaces.  One  recognizes  these  relations 
much  better  in  transverse  sections  of  the  corona  ciliaris  (PI.  VII,  2) ;  here, 
where  one  has  a  whole  series  of  ciliary  processes  in  cross-section  before  him, 
one  is  convinced  that  the  continuity  of  the  tissue  is  nowhere  broken,  but 
that  the  side  surface  is  only  wrinkled  and  the  ridges  somewhat  rounded  off. 

In  the  study  of  meridional  sections  the  error  is  not  infrequently  made  of  holding  the 
sims  or  an  incomplete  cut  through  the  process  to  be  the  entire  process.  One  can  protect 
himself  against  this  error  if  one  give  heed  to  the  pigmentation  of  the  epithelium;  the 
pigmentation  of  the  epithelium  is  very  much  less  upon  the  very  height  of  the  projec- 
tion, and  then  only  does  the  section  actually  go  through  the  ridge  of  the  ciliary  process. 


The  histologic  peculiarities  of  the  ciliary  body  are  easiest  made  clear 
by  a  comparison  with  the  posterior  zone  of  the  bulb : 


/ 

Middle  Zone 

Posterior  Zone 

Musculus  ciliaris  and  suprachorioidea 

Suprachorioidea 

3 

1  Vessel  layer  of  the  ciliary  body 

Vessel  layer  of  the  chorioidea 

l< 

/ 

Choriocapillaris 

Elastic  lamella 

Elastic  lamella         (   Lamina 

' 

Intermediary  connective  tissue 

1 

—                ^   vitrea 

Cuticular  lamella     \  chorioideae 

Cuticular  lamella 

■^ 

Pigment  epithelium  of  the  ciliary  body 

Pigment  epithelium  of  the  chorioidea 

2  a 

'S. 

Ciliary  epithelium 

Membrana  limitans  interna  ciliaris 

i  Layers  i  to  8 

Retina  -            

/   Membrana  hmitans  interna 

THE  CILIARY  BODY  iii 

a)    The  Uveal  Portion  of  the  Ciliary  Body 

I.    SUPRACHORIOIDEA   AND    CILIARY   MUSCLE 

If  one  follows  the  suprachorioidea  from  behind  forward  new  structural 
elements — smooth  muscle-fibers — appear  even  in  the  equatorial  region 
or,  at  times,  still  farther  posterior,  therefore  in  the  territory  of  the 
chorioidea.  They  are  grouped  in  bundles  singly  or  for  the  most  part 
branched  and  then  forming  three  or  more  rayed  stellate  little  figures 
(muscle-stars,  PI.  IV,  8). 

The  muscle-stars  are  flattened,  in  keeping  with  the  lamellar  structure 
of  the  whole  layer,  and,  therefore,  appear  only  as  very  slender  spindles  on 
meridional  section.  Their  true  form  and  their  distribution  in  .the  plane 
of  the  surface  can  only  be  studied  in  teased  preparations  of  the  chorioidea. 
They  are  disposed  over  both  surfaces  of  the  suprachorioidal  lamella, 
and  their  fibers  go  out  in  tufts  of  elastic  fibers  radiating  into  the  elastic 
plexus  of  the  neighboring  lamellae  (/). 

Sparse  and  separated  by  wide  interspaces  to  begin  with,  the  muscle- 
stars  become  more  numerous  and  more  distinct  as  one  approaches  the 
posterior  border  of  the  ciliary  muscle;  finally,  they  run  together  into 
polyhedral  meshes  (PI.  IV,  9,  st).  The  ciliary  nerves  bifurcate  in  the 
same  zone  and  form  a  wide-meshed  plexus  by  means  of  their  larger  and 
smaller  branches. 

The  ciliary  muscle  proper  (M)  begins  with  these  meshes  of  muscle- 
bundles.  A  closed  framework  soon  develops  out  of  this,  i.e.,  the  bundles 
disposed  in  various  planes  unite  with  one  another  and  the  muscle  mass 
becomes  thicker  and  thicker  through  further  branching.  This  structural 
framework  is  maintained  throughout  the  entire  ciliary  muscle,  only  the 
prevailing  direction  changes  gradually,  so  that  the  ciliary  muscle  falls 
into  various  portions. 

In  the  outer  layers  of  the  muscle  the  bundles  have  an  almost  pure 
meridional  direction,  i.e.,  the  meshes  of  the  framework  are  narrow  and 
drawn  out  in  the  meridional  direction;  therefore,  one  sees  almost  no 
intermediary  tissue.  The  respective  layers  of  muscle-bundles,  indeed, 
show  many  unions  in  the  direction  parallel  to  the  surface,  but  among  them- 
selves these  bundles  are  only  sparsely  united  with  one  another;  the 
suprachorioidal  lamellae  go  deeply  in  between  these  layers  and  their 
denser  pigmentation  can  be  followed  for  a  long  distance  into  the  muscle. 
This  portion  of  the  muscle  itself,  therefore,  appears  to  be  lamellated 
and  on  meridional  section  to  be  composed  of  many  longitudinally  dis- 
posed bundles.  It  is  called  the  meridional  portion,  from  the  direction 
of  the  bundles. 


112  ANATOMY  AND  HISTOLOGY  OF  THIO  HUMAN  EYEBALL 

Forward  the  thickness  of  this  portion  gradually  increases  up  to  one- 
third  of  the  maximum  of  the  entire  muscle.  At  the  very  fore  part  it 
again  thins  and  ends  at  the  scleral  roll:  the  muscle-fiber  bundles  go 
over  into  finely  fibrillated  connective  tissue,  likewise  of  a  meridional 
course;  this  tissue  presses  in  between  the  circular  bundles  of  the  scleral  roll 
and  probably  continues  farther  into  the  lamella  of  the  scleral  trabeculum. 
Occasionally  individual  cross-sections  of  bundles  are  seen  at  the  anterior 
end  of  the  meridional  portion. 

The  suprachorioidea  is  at  length  completely  lost  in  the  meridional 
portion  of  the  ciliary  muscle;  the  majority  of  the  lamellae  enter  the 
posterior  border  of  the  ciliary  muscle  along  with  the  muscle-stars,  and 
the  few  lamellae  which  lie  outside  this  gradually  go  over  into  the  muscle- 
bundles  from  the  outer  surface.  So  it  comes  about  that  the  most  anterior 
part  of  the  perichorioidal  space  is  entirely  free  of  suprachorioidal  lamellae. 
Not  infrequently  one  sees  a  short  muscle-bundle  inserted  into  the  lamina 
nsca  sclerae  at  the  very  front,  and,  therefore,  not  taking  part  in  the 
general  detachment  of  the  ciliary  body  (Sattler,  i88). 

The  radial  portion  succeeds  the  meridional  portion  inward.  In 
this  the  structure  of  the  framework  is  most  pronounced  and  the  section, 
therefore,  shows  an  irregular  net-form  marking.  Many  of  the  bundles 
appear  to  end  blind;  these  are  the  obliquely  coursing  bundles  whose 
continuation  falls  in  the  next  section.  The  interstices  of  the  framework 
are  filled  out  by  a  pretty  dense  connective  tissue,  which  carries  the  blood- 
vessels and  the  especially  numerous  nerve  branches,  and  in  heavily 
pigmented  eyes  also  contains  a  few  scattered  chromatophores.  This  por- 
tion received  its  name  from  the  fact  that  a  fan-like  radiation  of  the  surface 
was  read  out  of  a  divergence  of  the  bundles.  Yet  one  is  at  great  pains  to 
find  such  an  arrangement  of  the  musculature,  and  I  would  prefer  to  call 
it  the  reticulated  portion. 

The  radial  portion  attains  its  greatest  thickness  in  the  neighborhood  of 
the  inner  ledge  of  the  ciliary  body.  A  special  ending  is  not  to  be  ascribed 
to  this  portion,  for  the  framework  turns  back  into  itself.  For  example, 
this  closure  of  the  framework  by  means  of  many  circular  coursing  muscle- 
bundles  is  to  be  seen  in  the  form  of  a  net  along  the  inner  surface  of  the 
muscle  (F.  E.  Schultze,  196). 

The  union  with  adjacent  structures,  especially  the  surface  union  with 
the  vessel  layer  of  the  ciliary  body,  is  only  mediated  by  the  interstitial 
connective  tissue  of  the  muscle,  which  goes  directly  over  into  the  con- 
nective tissue  of  the  vessel  layer.  In  a  similar  way  the  anterior  end  of  the 
radial  portion  unites  with  that  portion  of  the  scleral  framework  which 
does  not  enter  the  scleral  roll:    the  connective  tissue  substratum  of  the 


THE  CILIARY  BODY  113 

trabeculum  of  the  iris  angle  goes  over  into  the  interstitial  connective 
tissue  of  the  muscle. 

On  the  side  of  the  anterior  chamber,  in  the  delimitation  of  which  just 
this  portion  of  the  muscle  takes  some  part,  the  muscle  framework  is  bor- 
dered by  a  thin  layer  of  connective  tissue  united  with  the  meshwork  of 
the  iris  angle,  on  the  one  hand,  and  with  the  iris  stroma  on  the  other  hand. 
This  layer,  as  well  as  the  most  anterior  portions  of  the  interstitial  tissue, 
is  especially  rich  in  fine,  wavy,  elastic  fibers;  these  are  pressed  together  at 
the  inner  border  of  the  scleral  roll,  for  instance,  and  radiate  out  from  here 
toward  the  root  of  the  iris  and  into  the  muscle.  Chamberward  the 
trabeculae  of  the  uveal  meshwork  course  to  the  root  of  the  iris  covered  by 
and  snugly  inclosed  in  this  layer.  In  general,  this  layer  varies  greatly 
in  its  density  and  composition  in  various  individuals. 

At  the  inner  ledge  of  the  ciliary  body  lies  the  circular  portion  of  the 
ciliary  muscle,  the  so-called  Mueller's  muscle,  named  after  its  discoverer, 
Heinrich  Mueller  (157).  As  its  name  indicates,  it  is  characterized  by  the 
circular  course  of  the  bundles  and,  therefore,  appears  as  a  group  of  cross- 
sections  in  the  meridional  section.  The  intermediary'  tissue  is  looser  than 
in  the  radial  portion ;  it  has  more  of  the  appearance  of  the  stroma  of  the 
iris,  whose  root  lies  just  in  front  of  this  portion. 

But  the  circular  portion  likewise  forms  only  a  part  of  the  muscle 
framework;  the  meshes  are  much  drawn  out  in  a  circular  direction,  and 
do  not  form  a  separate  part  independent  from  radial  portion.  Unions 
and  transitions  between  the  two  portions  occur,  so  that  in  most  sections 
one  cannot  state  exactly  how  far  the  circular  portion  extends.  In  any 
case  the  drawings  of  Iwanoff  (114)  do  not  correspond  to  the  actual 
conditions  in  this  respect. 

The  form  of  the  ciliary  muscle,  especially,  and  thereby  that  of  the 
entire  ciliary  body,  depends  upon  the  grade  of  development  of  the  circular 
portion.  In  the  myopic  tj-pe  the  circular  portion  is  very  weakly  developed 
or  fails  entirely;  in  the  hj-permetropic  t\-pe  it  is  very  strongly  developed, 
on  the  other  hand,  and,  therefore,  causes  the  inner  ledge  to  project  forward 
and  inward. 

It  at  once  comes  to  mind  that  these  various  types  of  the  ciliary  muscle  are  nothing 
more  than  the  expression  of  the  particular  requirements  of  accommodation  for  the 
refraction  concerned.  For  the  hyperope,  who  must  accommodate  for  distant  vision, 
certainly  uses  much  more  accommodation  in  his  whole  life  than  does  the  myope,  who 
can  work  even  at  near  objects  without  any  particular  accommodation.  But  this 
relationship  must  not  be  so  conceived  of  that  the  ciliary  muscle  of  the  hyperope  is 
thought  of  as  hypertrophic,  and  that  of  the  myope  as  atropic,  because  the  whole  mass 
of  the  muscle  in  the  myopic  tj^pe  may  be  even  greater  than  in  the  hyperope. 

Now  Heine  (90)  has  shown  that  the  eserinized  (contracted)  muscle  approaches  the 


114  ANATOMY  AND  HISTOLOGY  OF  THE  HUMAN  EYEBALL 

hy])eroi)ic  type,  the  atropinizcd  (relaxed)  muscle  the  myopic  type.  It  is,  therefore, 
conceivable  that  the  dilTerent  types  are  nothing  more  than  different  states  of  contraction 
of  the  muscle.  Against  this  it  is  to  be  argued  that,  as  a  rule,  there  is  no  occasion  for  the 
hyi^erope  to  accommodate  just  before  the  eye  comes  to  anatomic  study,  as  for  example 
in  the  last  hours  of  life  if  the  eye  is  removed  from  the  cadaver,  nor  during  the  anaesthesia 
if  it  is  enucleated  during  life.  Finally,  the  same  variations  are  found  even  in  the  new- 
born (see  chap.  xvii). 

■\11  things  considered,  one  must  conclude  that  the  form  of  the  ciliary  muscle  depends 
upon  its  length,  i.e.,  the  longer  the  muscle  from  whatever  cause,  the  more  the  myopic 
type  comes  to  e.xpression,  the  shorter  it  is,  the  more  the  hypermetropic  type  comes  out. 
The  transitory  changes  in  its  length,  by  contraction,  have  the  same  effect  as  the  perma- 
nent ones — those  which  develop  either  as  a  congenital  variation  or  as  a  result  of  the 
elongation  or  shortening  of  the  sagittal  diameter  of  the  eye. 

The  fibers  which  malce  up  the  ciliary  muscle  are  ordinary  smooth 
muscle-fibers,  although  the  nuclei  are  not  so  markedly  rod-form  but  more 
oval  than  is  generally  the  case. 

The  blood-vessels  peculiar  to  the  ciliary  muscle  lie  throughout  in  its 
interstitial  tissue  and  have  a  small  caliber.  The  larger  arteries  encoun- 
tered in  horizontal  sections  of  the  ciliary  muscle  are  the  main  branches 
of  the  long  posterior  cUiary  or  anterior  ciliary  arteries,  for  these  vessels 
course  through  the  muscle;  they  usually  lie  in  a  large  area  which  is  free 
from  muscle-fibers. 

Behind  the  root  of  the  iris  and  in  front  of  the  circular  portion  of  the 
muscle  or  in  it  one  encounters  the  cross-section  of  an  artery,  the  circidiis 
arteriosus  major,  in  all  meridional  sections ;  this  vascular  circle,  therefore, 
hardly  belongs  to  the  ciliary  muscle,  for  its  branches  supply  the  ciliary 
processes  and  mainly  the  iris.  Large  veins  are  not  found  in  the  ciliary 
muscle;  its  blood  is  partly  carried  backward  by  the  smaller  vessels  of  this 
sort  through  the  vessel  layer,  partly  outward  through  the  sclera;  the  latter 
vessels  are  the  venae  ciliares  anteriores;  they  also  take  care  of  the  outflow- 
from  the  canals  of  Schlemm  (see  p.  44). 

The  ciliary  nerves  branch  before  entering  the  ciliary  muscle,  form 
first  a  wide-meshed  plexus,  and  then,  continuing  to  branch,  pass  on 
through  the  muscle.  The  nerves  to  the  ciliary  muscle  itself,  as  well 
as  those  for  neighboring  parts,  are  given  off  from  this  plexus.  This  is 
particularly  true  of  those  for  the  iris  and  the  deeper  layers  of  the  corona. 
This  plexus  consists  mainly  of  medullated  fibers  and,  aside  from  larger 
ganglion  cells,  usually  contains  small  bipolar  ganglion  cells,  which  are 
in  all  probability  motor  cells  (H.  Mueller,  159;  Iwanoff,  115). 

There  are  but  few  contributions  extant  upon  the  finer  relations  of  the  nerves  of 
the  ciliary  muscle.  Agababow  (4)  has  found  motor  endings  in  the  muscle-fibers  in  the 
shape  of  straight  fibers,  or  fibers  dividing  at  sharp  angles,  by  the  aid  of  the  methyl-blue 


THE  CILIARY  BODY  iiS 

stain,  and  end  arborizations  with  pretty  large  branches  ending  by  nodular  expansions. 
These  lie  in  the  interstitial  connective  tissue  and  were  considered  to  be  sensor)^  endings 
ser\ing  the  muscle-sense.  Finally,  Agababow  found  the  so-called  reticular  plate,  i.e.,  an 
extremely  fine  fibrillar  net,  which  can  only  be  resolved  by  the  aid  of  the  oil  immersion 
system.     Yet  accurate  data  concerning  the  situation  of  this  layer  is  lacking. 


2.       THE    VESSEL   LAYER   OF   THE    CILIARY   BODY 

This  is  a  direct  continuation  of  the  vessel  layer  of  the  chorioidea,  for 
the  vortex  veins  carry  away  the  blood  not  only  from  the  chorioidea  but 
also  from  the  anterior  portion  of  the  uveal  tract,  especially  from  the  iris 
and  the  ciliary  processes.  Since,  however,  the  arterial  trunk  for  this  vas- 
cular region  runs  through  the  perichorioidal  space  and  the  ciliary  muscle, 
the  vessel  layer  of  the  ciliary  body  contains  only  veins  of  varying  caliber, 
with  the  exception  of  a  few  arteries  running  back  to  the  chorioidea. 
These  vessels  course  nearly  parallel  to  one  another  and  occasionally 
anastomose  at  narrow  angles  at  the  vortices;  therefore,  one  gets  only 
longitudinal  or  very  oblique  sections  of  these  vessels  on  meridional  cuts, 
on  the  other  hand,  almost  pure  cross-sections  on  transverse  cuts;  in  the 
orhiculus  ciliaris  these  vessels  are  broadened  out  into  what  is  scarcely 
more  than  a  single  layer,  i.e.,  narrow  and  wide  vessels  lie  on  the  same 

plane(Pl.  VII,  4,  5,G0- 

The  larger  elevations  of  the  ciliary  body,  the  ciliary  processes,  the 
sims,  the  plicae  clliares,  and  the  warts  in  the  valleys  are  due  to  local  thick- 
enings of  the  vessel  layer  with  a  corresponding  superimposition  of  the 
vessel  lamina.  The  ciliary  muscle  has  no  part  in  any  of  these  formations; 
its  inner  contour  courses  along  straight  beneath  the  elevations  (PL  VII, 
2),  as  the  transverse  section  shows.  In  particular,  each  ciliary  process 
(Pc)  conceals  a  richly  subdivided  framework  of  wide  capillaries  and 
small  veins  supplied  by  a  small  artery  from  in  front,  and  sends  several 
veins  backward  into  the  orhiculus  ciliaris. 

In  respect  to  its  histologic  structure,  the  vessel  layer  of  the  ciliary 
body  also  agrees  with  that  of  the  chorioidea,  except  that  the  chromato- 
phores  are  less  numerous  and  almost  completely  disappear  toward  the 
front  in  many  eyes.  The  ciliary  processes  only  exceptionally  contain 
chromatophores;  for  this  reason  the  collagenous  connective  tissue  comes 
out  more  prominently  and  is  denser  in  this  region.  In  the  anterior 
part  of  the  ciliary  processes  especially  (toward  the  iris  root),  this  tissue 
morphologically  assumes  a  sclerosed  appearance,  and  these  places  then 
stain  an  intense  red  by  Van  Gieson.  Fine  elastic  fibers  are  irregularly 
intermixed  with  the  collagenous  tissue. 


ii6  ANATOMY  AND  HISTOLOGY  OF  THE  HUMAN  EYEBALL 

3.      THE   ELASTIC   LAMELLA 

(PI.  vn,  4, 5,  ci) 

This  is  a  continuation  of  the  lameUa  of  the  same  name  in  the  lamina 
vitrea  chorioideae  and  forms  the  inner  limitation  of  the  vessel  layer  in  the 
orbiculus  ciliaris  and  posterior  part  of  the  corona  ciliaris.  In  the  orbiculus 
ciliaris  it  is  seen,  after  all  stains,  as  a  fine,  very  sharp  straight  line  in 
case  the  section  goes  perpendicular  to  the  surface,  for  this  lamella  courses 
absolutely  smooth  over  the  orbiculus  ciliaris. 

The  slight  wrinkhng  shown  in  PI.  VII,  4,  5  is  brought  about  by  a  detachment 
of  the  cihary  body  from  the  sclera. 

Naturally,  it  comes  out  still  more  plainly  after  elective  staining  for 
elastic  fibers,  e.g.,  orcein.     This  stain  resolves  it  into  a  net  of  elastic  fibers. 

With  the  transition  to  the  elevations  of  the  corona  ciliaris,  this  smooth- 
ness of  the  lamella  is  lost  and  it  therefore  disappears  in  the  unstained 
state  at  this  place,  for  it  is  usually  cut  obliquely  by  the  section.  In  the 
orcein  preparation  it  can,  however,  still  be  followed  as  far  as  the  middle 
of  the  corona.  The  fiber  net  constantly  becomes  looser  and  finally  rays 
off  in  irregular  bundles  into  the  collagenous  tissue  of  the  vessel  layer. 
This  lamella  cannot  therefore  be  demonstrated  over  the  anterior  declivities 
of  the  ciliary  processes. 

4.      THE   INTERLAMELLAR    CONNECTIVE    TISSUE 
(PI.  VII,  4,  5,  iB) 

This  is  evidently  a  continuation  of  the  delicate  layer  of  collagenous 
fibrillae  found  by  Wolfrum  (240)  between  the  two  layers  of  the  glass 
membrane  of  the  chorioidea  (cf.  p.  60).  While  special  methods  are 
necessary  to  demonstrate  the  collagenous  fibrillae  in  the  chorioidea, 
this  connective-tissue  layer  attains  such  a  thickness  in  the  ciliary  body 
that  ordinary  stains,  especially  Van  Gieson's  stain,  make  it  plainly  visible. 

For  example,  when  one  follows  the  lamina  vitrea  chorioideae  toward  the 
ciliary  body,  the  membrane  (which  so  far  has  appeared  as  a  single  mem- 
brane) splits  into  two  lamellae,  even  before  it  reaches  the  ora  serrata 
retinae;  the  outer  elastic  lamella  maintains  its  bow-string  course  still 
farther;  the  inner  cuticular  membrane,  however,  becomes  wavy  and 
separated  farther  and  farther  from  the  elastic  lamella  as  one  nears  the 
end  of  the  retina.  At  first  the  space  appears  empty  (by  Van  Gieson's 
stain),  then  there  appears  a  longitudinally  fibrillated  collagenous  tissue, 
which  becomes  thicker  and  firmer  in  the  orbiculus  ciliaris,  and  stains 
much  more  intensely  red  with  fuchsin  than  does  the  stroma  of  the  vessel 
layer.      Here   and    there,   the  interlamellar  tissue    contains    elongated 


THE  CILIARY  BODY  117 

nuclei,  but  no  blood-vessels.  In  surface  view,  the  wavy  fibrillae  show 
a  meridional  course,  as  is  well  depicted  by  Henle  (94). 

Sattler  (187)  found  a  special  capillary  system  in  the  posterior  part 
of  the  ciliary  body  and  the  most  anterior  portion  of  the  chorioidea; 
this  consists  of  narrow  capillaries  and  lies  inside  the  choriocapillaris. 
It  can,  therefore,  be  suspected  that  this  second  capillary  system  has  its 
seat  in  the  interlamellar  connective  tissue.  Since  Sattler  himself  found 
it  in  only  half  of  the  eyes,  it  may  well  be  only  an  abnormality;  I  have 
found  such  capillaries  in  only  a  few  otherwise  normal  eyes. 

Upon  the  cessation  of  the  elastic  lamella,  the  interlamellar  connective 
tissue  merges  with  the  vessel  layer. 

5.      THE    CUTICULAR   LAMELLA 

(Glass  membrane  of  the  ciliary  body  of  the  older  authors,  outer 

glass  membrane  of  the  pars  ciliaris  retinae,  184) 

(PI.  VII,  4,  S,  Cm) 

This  is  a  continuation  of  the  lamella  of  the  same  name  in  the  chorioidea 
and  is  of  the  same  structure  as  it;  it  covers  the  whole  uveal  portion  of 
the  ciliary  body  as  far  as  the  neighborhood  of  the  iris  root.  It  is,  in 
general,  very  thin,  and  possesses  a  greater  thickness  only  in  the  anterior 
third  of  the  ciliary  processes,  where  it  attains  the  thickness  of  Descemet's 
membrane  in  older  persons  (PI.  VIII,  11,  Cn).  In  this  location  it  is, 
moreover,  easiest  demonstrable,  especially  by  Van  Gieson's  stain;  the 
weakly  red-colored  membrane  then  stands  out  plainly  from  the  brUliant 
red  sclerosed  connective  tissue  of  the  vessel  layer. 

Yet  Wolfrum  (240)  is  of  the  opinion  that  the  clearer  layer  described  is  only  hyalin- 
ized  connective  tissue,  and  that  the  cuticulum  proper  is  here  as  thin  as  in  the  orbiculus 
ciliaris. 

The  difficulty  of  seeing  the  cuticular  lamella  in  the  other  parts  of  the 
ciliary  body  is  much  less  due  to  the  thinness  of  the  membrane  than  it  is  to 
the  numerous  uneven  areas  on  the  inner  surface  of  the  ciliary  bod}' ;  these 
are  formed  in  part  by  its  union  with  the  interlamellar  connective  tissue. 
The  irregularities  consist  of  ridges  of  varying  height  and  size ;  they  show 
many  horn-like  branchings  and  often  run  together  into  a  closed  network, 
so  that  the  inner  surface  of  the  ciliary  body  has  a  honeycombed  appear- 
ance. Heinrich  Mueller  (156)  first  accurately  described  this  structure 
and  it  is  called  the  reticulum  of  Heinrich  Mueller  after  him. 

Surface  preparations  are  absolutely  necessary  for  the  study  of  the  reticulum; 
only  in  this  way  does  the  network  come  out  properly  and  one  who  has  once  seen  such  a 
surface  preparation  will  never  have  the  idea  that  the  reticulum  is  brought  about  by 
a  wrinkling  of  the  inner  surface  from  the  contraction  of  the  ciliary  muscle. 


ii8  ANATOMY  AND  HISTOLOGY  OF  THE  HUMAN  EYEBALL 

For  making  such  a  ])reparation  cadaver-eyes  are  best;  the  epithelial  covering 
of  the  ciliary  body  is  then  more  easily  detached  from  the  cuticular  lamella,  for  the 
cadaverous  degeneration  only  destroys  the  protoplasmic  parts  and  leaves  the  more 
resistant  glass  membranes  unchanged.  The  retina  is  bluntly  detached  at  the  ora 
serrata,  the  pigment  epithelium  and  all  of  the  layers  lying  inside  then  comes  away 
with  it,  and  if  a  few  remnants  of  pigment  remain  behind  in  the  deeper  meshes  it  does 
not  interfere  with  the  recognition  of  the  reticulum.  One  then  turns  the  preparation 
over  and  deftly  removes  the  ciliary  muscle  and  the  vessel  layer.  The  inner  surface 
side  is  then  laid  up  and  the  preparation  is  mounted  in  glycerin  or  stained  with  Mallory's 
hematoxylin  and  mounted  in  Canada  balsam. 

In  this  way  one  gets  a  general  view  of  the  whole  orbiculus  ciliaris.  The  corona, 
alone,  does  not  permit  the  making  of  such  a  preparation,  because  the  folding  of  the 
surface  is  too  marked;  this  zone  is  better  studied  in  sections. 

In  general,  three  varieties  of  the  reticulum  with  respect  to  height  of 
ridges  and  width  of  meshes  can  be  made  out;  naturally,  these  are  not 
sharply  separated  from  one  another. 

1.  The  larger  meshes  (PL  VII,  4)  are  rounded  or  polygonal,  and  have 
a  width  of  40  to  50  mu  and  a  depth  of  as  much  as  40  mu;  the  ridges  (/) 
are  thick  and  high,  often  thickened  at  the  free  border  and  finely  striated 
on  the  surface. 

2.  The  small  meshes  (PL  VII,  3,  5)  have  only  about  one-half  or 
one-third  the  diameter  of  the  large  meshes  and  are  very  much  more 
irregular;  the  ridges  are  low  and  narrow. 

3.  Among  these  appear  ridges  characterized  by  special  thickness, 
height,  and  striation;  in  general,  they  have  a  meridional  course;  the 
immediately  adjacent  meshes  are,  therefore,  very  deep.  These  high  ridges 
go  over  into  low  and  narrow  ridges  at  the  sides  and  ends,  and  the  whole 
figure  thereby  takes  on  a  branched  appearance  or  has  similarity  to  a  bone 
corpuscle  (PL  VII,  3;  the  darkest  portion  of  the  reticulum). 

The  reticulum  extends  over  nearly  the  entire  inner  surface  of  the  ciliary 
body,  although  a  narrow  strip  in  front  of  the  ora  serrata  and  the  ridges 
of  the  ciliary  processes  are  practically  free  from  it. 

Starting  in  front  of  the  ora  serrata  one  usually  encounters  large  meshes 
right  where  the  reticulum  as  a  whole  begins.  Not  infrequently  they  are 
closed  off  into  a  girdle,  which  is  then  visible  even  with  a  loupe,  and  copy 
the  form  of  the  ora  serrata  (cf.  p.  108). 

Furthermore,  the  striae  ciliarcs  are  sometimes  formed  by  several  rows 
of  large  meshes.  The  small  meshes,  or  a  zone  with  an  indistinct  reticulum, 
follow  immediately  upon  the  large  meshes  in  front.  In  any  case  the 
small  meshes  are  encountered  in  the  fore  part  of  the  orbiculus  as  a  closed 
zone,  and  in  this  are  the  high  and  thick,  bone-corpuscle-like  ridges  (PL 
VII,  3)- 


THE  CILIARY  BODY  119 

The  reticulum  again  becomes  less  plain  over  the  corona  ciliaris,  in 
particular  in  the  ciliary  valleys  in  front,  inward  along  the  sides  of  the 
processes,  and  disappears,  as  above  stated,  over  the  ridges  of  the  ciliary 
processes,  i.e.,  in  so  far  as  they  show  a  lighter  color,  or  is  reduced  to 
wholly  insignificant  irregularities. 

On  section  the  reticulum  appears  only  as  an  angular  contour  when 
weakly  developed — something  like  the  silhouette  of  a  far-off  mountain 
chain.  When  well  developed,  however,  narrow  projections  stand  out, 
bordered  by  parallel  smooth  sides  with  rounded  edges,  or  (in  the  larger 
ridges)  often  ending  with  a  thickened  rolling  margin.  The  height  of  the 
ridges  varies  much  (PI.  VII,  4) ;  as  a  rule,  the  heavier  ridges  are  also  the 
higher.  The  inner  surface  of  the  orbiculus  also  takes  on  a  slight  uneven- 
ness  from  the  varying  development  of  the  reticulum.  In  this  way  a 
closed  large-meshed  zone  forms  an  even,  wall-like  thickening;  the  striae 
ciliaris,  likewise,  stand  out  somewhat  above  the  level  of  the  remainder  of 
the  inner  surface,  and  the  bone-corpuscle-like  ridges  behind  the  corona 
ciliaris  form  correspondingly  elongated  hummocks. 

The  thin  ridges  (PI.  VII,  5)  often  consist  of  only  the  cuticular  lamella; 
the  thicker  ones,  however,  contain  a  connective  tissue  arising  from  the 
layer  at  the  base  of  the  interlamellar  connective  tissue  (PL  VII,  4) ; 
it  is  this  which  lends  the  reported  striation  to  the  surface  view.  The 
layers  of  the  ciliary  body  lying  farther  outward  take  no  part  in  the  forma- 
tion of  the  reticulum;  the  elastic  lamella,  in  particular,  courses  smoothly 
beneath  the  ridges. 

h)    The  Epithelial  Covering  of  the  Ciliary  Body 
{Pars  ciliaris  retinae  of  the  authors) 

6.      PIGMENT   EPITHELIUM   OF   THE    CILIARY   BODY 
(PI.  VII,  2,  4,  5,  P) 

This  is  the  direct  continuation  of  the  pigment  epithelium  of  the 
chorioidea  and,  like  this,  consists  of  a  single  layer  of  pigmented  epithelial 
cells.  Only  in  so  far  as  this  layer  is  compelled  to  lie  in  the  deepenings 
of  the  reticulum,  does  there  come  about  a  heaping  up  of  cells  and  this  is 
without  loss  of  the  principle  of  a  single  layer.  The  inner  ends  (the  heads) 
of  the  cells  show  a  straight  border,  the  pigment  processes  characteristic  of 
the  chorioidal  pigment  epithelium  are  absent;  colorless  cement  ridges 
are  not  visible  in  the  pigment  epithelium  of  the  ciliary  body;  the  cell 
borders,  like  the  cell  muscles,  can  only  be  seen  in  bleached  sections.  The 
pigment  consists  of  larger  darker  granules,  which  are  throughout  rounded; 


120  ANATOMY  AND  HISTOLOGY  OF  THE  HUMAN  EYEBALL 

therefore,  Ihc  pigment  epithelium  of  the  ciliary  body  as  a  whole  api)ears 
darker  and  blacker  than  does  that  of  the  chorioidea. 

The  form  of  the  cells  changes  in  various  parts  of  the  ciliary  body. 
Where  the  inner  surface  is  smooth,  as  just  in  front  of  the  ora  serrata,  the 
cells  are  short  and  cylindrical,  some  6  mu  broad  and  i8  to  23  mu  high; 
the  nucleus  is  oval  and  placed  with  the  long  axis  at  right  angles  to  the 
inner  surface  of  the  ciliary  body.  The  large  meshes  of  the  reticulum  are 
filled  out  by  pigment  (PL  VII,  4),  the  cell  layer  as  a  whole  goes  down 
into  the  depression,  and  its  cells  take  on  an  irregular  polyhedral  form. 
These  evaginations  of  the  pigment  epithelium  do  not  possess  a  lumen,  only 
a  funnel-form  depression  on  the  inner  surface  of  the  pigment  epithelium. 
The  layer  as  a  whole  undergoes  an  appreciable  thickening  by  these 
evaginations  (as  much  as  60  to  80  mu)  and  seems  just  so  much  darker  on 
the  surface  view  than  do  the  smooth  portions  (darker  girdle  in  front  of  the 
ora  serrata  and  the  striae  ciliares). 

The  small  meshes  (PI.  VII,  5)  are  too  narrow  for  the  pigment 
epithelium  to  lie  in  them  as  a  layer;  they  are,  therefore,  only  filled  out 
by  correspondingly  displaced  cells,  and  here  one  often  sees  the  base  of  the 
cell,  i.e.,  the  side  of  it  turned  toward  the  uveal  portion,  free  from  pigment 
granules.  The  pigment  epithelium  over  the  ridges  of  the  ciliary  processes 
becomes  significantly  lower  (PL  VIII,  11,  P)  (height  of  the  cell  10  to  15  mu), 
the  cells  come  to  have  more  breadth  than  height,  and  the  nuclei  stand 
obliquely.  The  pigmentation,  likewise,  decreases  appreciably,  so  that 
one  can  recognize  borders  and  nuclei  even  without  depigmentation 
(PL  VII,  2).     This  explains  the  whitish  color  of  the  ciliary  ridges. 

The  union  between  the  individual  pigment  epithelial  cells  is  possibly 
no  more  firm  than  it  is  in  the  territory  of  the  chorioidea.  But  the 
reticulum  brings  about  a  considerable  increase  of  the  surface  and  effects 
a  firmer  fixation  (anchoring)  of  the  pigment  epithelium  by  means  of  its 
many  projections.  As  a  matter  of  fact,  the  pigment  epithelium  cannot 
be  removed  wholly  intact,  even  in  a  macerated  eye;  pigment  remnants 
remain  behind,  especially  in  the  large  meshes. 

The  outer  surface  of  the  pigment  epithelium  everywhere  forms  a  perfect 
mould  of  the  inner  surface  of  the  cuticular  lamella.  This  is  best  seen  in 
sections  of  cadaver-eyes  in  which  even  a  desquamation  of  the  epithelium 
has  begun.  Over  the  crests  of  the  ridges,  too,  the  epithelium  stands 
away  and  the  ridges  of  the  reticulum  never  reach  through  the  entire  thick- 
ness of  the  pigment  epithelium;  it  never  comes  in  contact  with  the  next 
layer,  the  ciliary  epithelium. 

Aside  from  the  evaginations  of  the  pigment  epithelium  brought  about 
by  the  reticulum,  there  are  other  evaginations  very  similar  to  those  of 


THE  CILIARY  BODY  121 

genuine  glands.  These  are  the  flask-form  epithelial  plugs  engirt  by  the 
cuticular  element — lamellae  which  pass  through  the  interlamellar  con- 
nective tissue  and  the  elastic  lamellae  and  with  their  thickened  ends  reach 
as  far  as  the  vessel  layers.  A  lumen  is  not,  however,  demonstrable  in 
these  structures;  they  therefore  lack  an  essential  morphologic  element  of  a 
gland.  Such  "glands"  are  found  in  the  anterior  part  of  the  orbiculus 
especially,  but  even  here  only  scatteringly. 

Whether  or  not  one  should  consider  these  structures  glands,  as  does  Treacher  Collins 
(226),  seems  to  me  to  be  a  matter  of  no  importance.  There  is  no  doubt  that  the  cOiary 
body  secretes  the  aqueous,  but  we  have  no  warrant  that  this  function  is  reserved  for 
the  reported  "glands"  and  that  the  rest  of  the  inner  surface  of  the  ciliary  body  takes 
no  part  therein. 

7.      THE    CILIARY   EPITHELIUM 
(PL  VII,  2,  4  5,  CE) 

This  also  forms  a  simple  smooth  layer  of  cells,  aside  from  several  folds 
in  the  anterior  part  of  the  orbiculus.  The  protoplasm  of  these  cells  is, 
in  general,  free  from  pigment;  pigment  is  found  in  the  ciliary  epithelium 
only  anteriorly  in  the  neighborhood  of  the  iris  root. 

This  layer  corresponds  to  the  entire  retina  sensu  strictiorl,  with  the 
exception  of  the  membrana  limitans  interna;  one  can  convince  himself  of 
this  best  in  the  eye  of  the  newborn,  where  the  transition  of  the  retina  into 
ciliary  epithelium  is  still  a  wholly  gradual  one  (PI.  IX,  i).  Its  union 
with  the  pigment  epithelium  is  a  great  deal  more  firm  in  any  case  than  is 
that  of  the  retina  with  the  pigment  epithelium  of  the  chorioidea,  for  arti- 
ficial and  post-mortal  detachment  stop  at  the  border  of  the  retina.  This 
union  is  apparently  effected  by  a  cement  substance  like  that  uniting  the 
individual  ciliary  epithelial  cells  to  one  another;  but  I  cannot  make  out  a 
special  membrane  corresponding  to  a  continuation  of  the  membrana 
limitans  externa.  Not  infrecjuently  one  sees  small  vacuoles  between  the 
pigment  epithelial  cells  and  ciliary  epithelium  in  cadaver-eyes,  and  here 
the  points  in  which  they  are  united  are  drawn  out  into  pedicles. 

The  form  of  the  cells  varies  from  that  of  a  cylinder  to  a  cube,  and,  in 
general,  their  height  increases  from  behind  forward. 

Immediately  in  front  of  the  overhanging  border  of  the  retina,  some 
particularly  long-drawn-out  fiber-like  cells  appear,  as  reported  on  p.  86; 
otherwise  the  cells  in  the  most  posterior  parts  of  the  orbiculus  are  6  to 
9  mu  broad,  and  some  30  mu  high,  therefore  markedly  cylindrical. 
The  elongated  nuclei  lie  nearer  the  outer  ends  of  the  cells.  Although 
most  of  the  cells  are  straight  or  inclined  slightly  forward,  or  bowed, 
small  groups  of  cells  show  a  distinct  bend  backward.     A  peculiar  crossing 


122  ANATOMY  AND  HISTOLOGY  OF  THE  HUMAN  EYEBALL 

of  the  cells  arises  in  this  way,  a  microscopic  chiasma,  so  to  speak  (drawn 
by  von  Ebner,  120).  When  a  closed  large-mesh  zone  is  formed  in  the 
reticulum  of  H.  Mueller,  the  cells  in  this  zone  are  still  higher  (40  to  60  mu) 
and  the  roll  is  thereby  heightened. 

The  inner  ends  of  the  cells  show  a  form  varying  much  with  the  direc- 
tion of  the  cells;  the  inner  surface  of  the  whole  layer  is  not  always  smooth, 
and  frequently  shows  irregular,  tooth-like  projections  (PI.  VII,  4). 
According  to  Addario  (3),  the  inner  ends  of  the  cells  go  over  into  finely 
striated  granular,  pointed  processes,  2  to  3  times  as  long  as  the  cell  itself, 
and  broaden  out  upon  the  inner  surface  of  the  ciliary  epithelium,  partly 
in  the  direction  of  the  corona,  partly  toward  the  ora  serrata. 

The  union  between  the  cells  here  is,  moreover,  often  a  loose  one; 
clefts  and  vesicular  spaces  are  found  between  the  cells,  especially  in  those 
eyes  which  show  cystoid  degeneration  of  the  retina.  This  spacing  up  of 
the  ciliary  epithelium  is  apparently  analogous  to  the  so-called  degeneration 
of  the  retina. 

Even  in  the  anterior  parts  of  the  orbiculus  the  cells  are  still  cylindrical, 
although  much  lower  than  in  the  posterior  parts,  and  very  unequal  in 
height,  for  ridges  of  the  membrana  limitans  interna  ciliaris  (cf.  p.  124) 
are  found  here  and  the  cells  sink  down  in  between  them.  These  ridges 
occasion  irregularities  of  the  ciliary  epithelium,  because  of  their  increased 
thickness,  especially  in  the  eyes  of  older  persons;  indeed,  it  may  come 
about  that  the  epithelium  between  two  ridges  is  actually  folded  (PL 
VII,  5). 

When  one  studies  such  eyes  in  meridional  section,  the  ridges  come  out 
less  plainly;  on  the  other  hand,  the  irregularities  of  the  ciliary  epithelium 
are  more  difficult  to  understand :  the  cell  borders  disappear  for  a  distance 
(longitudinal  section  of  a  ridge),  then  the  ciliary  epithelial  cells  are  piled 
up  on  top  of  each  other  for  a  stretch  (longitudinal  section  of  a  fold).  Only 
transverse  sections  of  the  orbiculus  give  us  a  clear  picture. 

Over  the  crests  of  the  ciliary  processes  the  cells  are  cubical  (12  to 
15  mu  broad,  10  to  15  mu  high),  and  the  nuclei  are  rounded  (PI.  VIII, 
11;   the  less  pigmented  cells). 

As  one  follows  the  ciliary  epithelium  still  farther  toward  the  iris  root, 
some  pigment  appears  even  in  the  anterior  part  of  the  ciliary  processes, 
first  in  the  portion  lying  outside  of  the  nucleus.  The  protoplasm  itself  does 
not  stain  at  all  in  this  portion;  the  pigment  granules  are  much  scattered. 
At  the  same  time,  this  pigmentation  is  sufficient  to  make  the  anterior 
declivity  of  the  ciliary  crests  appear  notably  darker  than  the  posterior  part 
(PI.  IV,  10).     In  the  neighborhood  of  the  iris  root  the  pigmentation 


THE  CILIARY  BODY  123 

rapidly  increases  and  an  enlargement  of  the  cells  takes  place  at  the  same 
time  (PI.  VIII,  11). 

The  so-far  unpigmented  ciliary  epithelium  thus  undergoes  a  transi- 
tion into  a  pigmented  epithelium,  which  is  uniform  over  all  parts  of  the 
iris  and  goes  over  onto  this  without  any  line  of  demarkation.  In  this 
way  the  border  between  the  ciliary  epithelium  and  the  pigment  epithelium 
of  the  iris  does  not  fall  at  the  root  of  the  iris  but  somewhat  behind  it. 
This  is  quite  without  significance  for  the  function  of  the  eye,  and  in  some 
other  respects ;  its  significance  lies  solely  in  that  it  demonstrates  ad  oculos 
that  the  pigment  epithelium  of  the  iris  belongs  to  the  tunica  interna, 
that  is  to  say,  to  the  inner  layer  of  the  optic  vesicle,  and  not  to  the  outer, 
as  one  would  be  led  to  believe  from  its  pigment  content. 

That  which  was  said  about  the  border  portions  of  the  retina  holds  true  in  an 
analogous  way  for  the  ciliary  epithelium;  it  is  often  extremely  difficult  to  draw  the 
line  between  normal  relations  on  the  one  hand  and  artefacts  and  pathologic  conditions 
on  the  other  hand.  It  is  certain  that  the  ciliary  epithelium  has  a  greater  smoothness 
and  regularity  in  young  eyes  than  in  old.  Certain  it  is  too,  that  the  lens  usually 
shrinks  considerably  in  the  hardening  fluid  (about  i  mm  in  the  equatorial  diameter), 
and  that  the  shrinking  causes  a  greater  tension  upon  the  zonula  fibers  and  thereby 
draws  upon  the  ciliary  epithelial  cells.  Under  such  circumstances  special  care  is 
necessary  in  forming  judgment  concerning  variations  in  the  ciliary  epithelium. 


8.      THE   MEMBRANA   LIMITANS   INTERNA    CILIARIS 

(Inner  glass  membrane  of  the  pars  ciliaris  retinae,  184) 
(PI.  VIII  2,  4,  s,  Li) 

A  structureless,  connected  layer  is  to  be  made  out  on  the  inner  surface 
of  the  ciliary  epithelium,  at  least  in  the  adult  eye;  it  is  plainly  differen- 
tiated from  the  protoplasm  of  the  ciliary  epithelial  cells  by  its  staining 
reaction  and  homogeneous  structure.  This  membrana  limitans  interna 
is  probably  best  looked  upon  as  a  cuticular  formation  of  the  ciliary  epithe- 
lium and  corresponds  in  its  position  to  the  membrana  limitans  interna 
retinae.  In  eyes  fixed  while  still  warm,  it  adjoins  the  protoplasm  of  the 
ciliary  epithelium  without  any  intervening  space;  in  cadaver-eyes,  how- 
ever, it  is  detached  from  this,  and  the  inner  space  seems  to  be  filled  out 
with  a  clear  coagulated  fluid. 

This  detachment  does  not  take  place  along  the  whole  extent  of  the 
membrane  but  in  sections,  and  forms  relatively  short,  often  very  regular 
bows  or  arcades  (PI.  VII,  2).  In  the  places  where  two  arcades  meet  this 
membrane  sinks  down  in  between  the  epithelial  cells  in  the  form  of  a 
ridge,  and  is  thereby  more  firmly  fixed. 


124  ANATOMY  AND  HISTOLCXIV  OF    I'HK  HUMAN  EYEBALL 

Such  ridges  (PI.  VII,  5,  /)  arc  most  plainly  developed  in  the  anterior 
part  of  the  orbiculus  ciliaris  and  have  here  a  meridional  direction,  as  com- 
parison of  meridional  and  transverse  sections,  and  especially  teased  prepa- 
rations, show.  The  ridges  do  not  course  entirely  straight,  however,  but 
zig-zag,  and  often  give  off  short  lateral  branches  at  the  ends.  Yet  a 
regular  net-formed  union  does  not  come  about,  as  in  the  reticulum  of 
Heinrich  Mueller  to  which  this  ridge  system  has  similarity  in  its  relation 
to  the  epithelium.  Neither  the  ridges  of  the  membrana  limilans  interna 
ciliaris  nor  those  of  the  cuticular  lamella  project  as  far  as  the  outer 
border  of  this  layer. 

On  the  free  surface  the  membrane  is  very  thin,  often  scarcely  measur- 
able; the  ridge  may,  however,  attain  considerable  thickness.  In  young 
individuals,  of  course,  the  cross-section  of  such  a  ridge  appears  narrow 
and  has  parallel  sides,  and  can  be  very  easily  overlooked.  In  older  people, 
however,  the  cross-section  appears  distended  like  a  flask  and  not  infre- 
quently shows  lobulated  branchings.  These  thickenings  of  the  ridges, 
apparently  increasing  with  years,  call  forth  irregularities  and  even  foldings 
of  the  ciliary  epithelium.  Each  ridge  can  be  considered  a  fold  or  duplica- 
tion of  the  membrana  limitans  interna  ciliaris  whose  two  leaves  lie  closely 
opposed  and  are  fused  to  one  another;  often  one  sees  a  delicate  separating 
line  and  at  times  fine  zonula  fibers  lying  therein  (cf.  chap.  xii). 

Surface  preparations  of  this  membrane  can  only  be  obtained  from  fresh  eyes, 
and  even  then  very  many  cells  still  cling  to  the  sides  of  the  ridges,  so  that  only  a  few 
places  give  a  pure  picture  of  the  ridges  m  such  a  preparation.  I  have  sketched  this 
appearance  {loc.  cit.,  184;  Text  Fig.  10).  Transverse  sections  through  the  orbiculus 
are  the  most  instructive;  meridional  sections  cut  the  ridges  longitudinally,  give  only  a 
very  indistinct  picture  and,  therefore,  easily  permit  misinterpretations. 

Ridges  are  likewise  present  in  the  ciliary  valleys,  yet  their  arrangement 
is  less  regular.  On  the  crests  of  the  ciliary  processes  they  go  over  into 
a  circular  direction,  i.e.,  they  course  obliquely  over  the  crests  and  thereby 
give  rise  to  an  appearance  recalling  that  of  the  large  intestine  and  its 
haustra. 

The  membrana  limitans  interna  ciliaris  is  easily  demonstrable  in  all 
these  places,  and,  moreover,  can  be  prepared  anatomically.  In  the  most 
posterior  portions  of  the  orbiculus,  however,  it  is  otherwise;  I  am  in 
doubt  whether  or  not  a  special  membrane  is  present  here.  The  ciliary 
epithelium  certainly  appears  very  plainly  set  off  from  the  vitreous  in 
section,  but  more  than  a  contour  between  the  two  I  cannot  see.  It  is 
not  represented  by  the  anatomic  preparation  which  I  once  considered 
proof  of  a  connection  between  the  membrana  limitans  interna  ciliaris 
and  the  membrana  limitans  interna  retinae  {loc.  cit.,  184;    Text  Fig.  9), 


THE  IRIS  125 

because  this  contained  a  part  of  the  vitreous  base  as  well.  According  to 
Addario  (3),  it  is  the  superimposed,  imbricated,  inward  prolongations 
of  the  ciliary  epithelial  cells,  which  give  rise  to  the  appearance  of  a 
membrane.  The  ends  of  these  processes  would  go  over  into  vitreous 
fibrillae;  such  a  consideration  excludes  the  conception  of  a  border  mem- 
brane. Wolfrum,  on  the  other  hand,  holds  that  there  is  a  delicate  basal 
membrane  in  this  situation. 

The  limitans  interna  retinae  is  also  at  times  very  indistinct  in  the 
neighboring  border  portions  of  the  retina.  I  have  described  a  very 
peculiar  proliferation  of  the  Mueller  supporting  fibers  into  the  vitreous 
in  a  case  of  probable  congenital  keratoconus  (185).  As  it  later  developed, 
Iwanoff  had  already  described  and  depicted  similar  changes  in  the  year 
1865  (113),  and  since  then  I  have  found  this  condition  indicated  in  some 
other  eyes.  The  proliferation  does  not  appear  to  have  a  pathologic 
significance;  it  only  indicates  that  the  delimitation  of  the  supporting 
tissue  of  the  peripheral  portions  of  the  retina  from  that  of  the  vitreous 
is  a  defective  one,  and  the  developmental  history  of  the  vitreous  gives  the 
key  to  this. 

The  limitans  interna  retinae,  like  the  limitans  interna  ciliaris,  has, 
therefore,  the  tendency  to  fade  away  toward  the  most  posterior  zone 
of  the  orbiculns  ciliaris.  Whether  it  actually  does  so  or  not  or  is  only 
there  reduced  to  a  membrane  so  thin  that  it  vanishes,  I  leave  unsettled. 
Probably  this  is  not  uniform  in  all  eyes  and  at  all  ages. 

In  front  the  limitans  interna  ciliaris  can  be  followed  as  far  as  the 
ciliary  epithelium  remains  unpigmented,  in  any  case. 

Wolfrum  (242)  will  only  admit  a  limitans  in  those  places  where  no 
zonula  fibers  are  given  off.  This  is  only  a  logical  sequence  of  his  views 
concerning  the  relations  of  the  zonula  fibers  to  the  ciliary  epithelial  cells 
(cf.  chap.    xii). 

CHAPTER  X.     THE  IRIS 

Severed  from  its  connections,  the  iris  forms  a  circular  plate  closely 
resembling  a  diaphragm,  such  as  one  uses  in  optical  instruments;  yet, 
in  fact,  it  is  not  broadened  out  into  a  plane,  but  forms  the  mantling 
surface  of  the  base  of  a  low  trunkated  cone,  because  the  lens  presses  the 
central  portion  forward. 

The  outer  (peripheral)  margin  of  this  plate,  that  united  to  the  ciliar}- 
body,  is  called  the  ciliary  border  {margo  ciliaris)  or  iris  root,  the  circular 
opening  in  the  middle  is  called  the  pupil,  and  the  limiting  margin  the  pupil- 
lary border  {margo  pupillaris).  The  pupil  does  not  lie  exactly  in  the 
center  but  slightly  to  the  nasal  side. 


126  ANATOMY  AND  HISTOLOGY  OF  THE  HUMAN  P:YEBALL 

The  diameter  of  the  ciliary  border  (considered  as  a  circle)  is  the  same 
as  that  of  the  cornea  (some  12  mm)  and  is  unchanging;  the  width  of  the 
pupil  changes  very  considerably  during  life,  and  likewise  the  breadth  of 
the  iris  (considered  as  a  ring).  In  the  thickest  portions,  the  iris  measures 
some  o .  4  to  o .  6  mm,  depending,  naturally,  upon  the  state  of  contraction  of 
the  pupil.  Since,  too,  the  iris  thins  out  appreciably  toward  both  borders, 
one  can  only  differentiate  two  surfaces,  an  anterior  and  a  posterior. 

With  the  exception  of  the  most  extreme  periphery,  the  anterior  surface  of  the  iris 
can  easily  be  seen,  even  during  life.  This  is,  indeed,  actually  covered  by  sclera  only 
above  and  below,  but,  since  the  cornea  magnifies  the  image  of  the  iris,  the  outermost 
periphery  is  necessarily  invisible  even  upon  this  ground.  One  can,  therefore,  see  the 
anterior  surface  of  the  iris  in  its  whole  extent  only  in  an  anatomic  preparation.  This 
can  easily  be  made  by  tearing  the  iris  away  from  the  ciliary  body,  for  just  as  occasion- 
ally after  injuries  in  life,  the  dead  iris  tears  away  from  the  ciliary  body  because  this 
place  is  the  weakest  of  the  whole  anterior  part  of  the  uveal  tract. 

An  iris  prepared  free  in  this  way  can  be  spread  out  fiat  and  studied  out  under 
moderate  magnification  just  as  it  is.  Strong  magnification  cannot,  moreover,  be  made 
use  of,  partly  because  of  the  thickness  of  the  iris,  partly  because  of  the  necessity  of 
study  by  reflected  light. 

In  the  study  of  the  iris  from  in  front  (PI.  VIII,  i)  with  the  pupil 
moderately  wide,  one  sees  a  fine,  black-brown  seam  (Ps)  at  the  pupil- 
border;  this  breaks  up  into  a  chain  of  little  beads  under  the  loop.  As 
readily  understood,  this  seam  is  best  seen  in  a  blue  iris,  and  especially 
when  the  lens  behind  it  is  clouded. 

Out  of  the  often  very  complicated  markings  of  the  anterior  surface  of 
the  iris  there  comes  first  a  zig-zag  line,  located  about  i .  5  mm  from  the  pupil 
when  it  is  of  middle  width.  This  angular  line  (Z),  usually  called  the 
small  circle  (or  ruffle,  Krueckmann,  125),  separates  the  anterior  surface 
into  two  zones:  an  inner,  the  pupillary  zone,  annul  us  iridis  minor  Merkel 
{Pz),  and  an  outer,  the  ciliary  zone,  annul  us  iridis  major  Merkel  (Cz). 
The  two  zones  vary  in  structure  and  also  at  times  in  their  color. 

The  name  small  circle,  the  circulus  iridis  minor,  really  denotes  the  anastomosing 
circle  lying  in  this  situation  in  the  vessel  system  of  the  iris.  But  that  which  one  sees 
on  the  anterior  surface  of  the  iris  is  not  the  anastomosing  circle  itself  but  only  contains 
it.  If  one  wishes  to  learn  the  real  significance  of  this  border  line,  those  cases  must  be 
drawn  upon  in  which  remnants  of  a  persistent  fetal  pupillary  membrane  are  present. 
One  then  recognizes  that  such  threads  go  out  from  the  angular  line,  bridge  over  the 
pupillary  zone,  and  so  extend  to  the  lens  capsule.  The  zig-zag  line  is,  therefore,  the 
insertion  point  of  the  fetal  pupillarj'  membrane  (Lohmann,  144),  or  rather  its  physio- 
logic remnant. 

The  pupillary  zone  as  a  whole  slopes  pretty  uniformly  away  from  the 
pupil-border.  It  carries  numerous  ridges  and  trabeculae,  which  go  out 
from  the  angular  line,  and  in  its  neighborhood  form  a  net-form  marking. 


THE  IRIS  127 

more  radially  striated  toward  the  pupillary  border.  Larger,  sharply 
limited  cavities,  the  pupillary  crypts  (k,),  arise  by  a  wide  separation  of 
the  individual  trabeculae  which  go  to  make  up  the  net,  usually  just  at 
the  angular  line;  net-like  trabeculae  are  again  united  on  the  floor,  or 
visible  only  as  wavy  radial  stripes. 

As  a  rule,  a  crypt  appears  darker  than  the  neighborhood,  partly  be- 
cause the  floor  is  thrown  into  a  shadow  by  the  steep  borders,  partly 
because  a  thinner  layer  of  connective  tissue  is  present  and  the  darker  color 
of  the  back  surface  of  the  iris  shimmers  through. 

In  general,  the  number,  size,  and  position  of  the  crypts  is  subject  to 
considerable  variation.  At  times  isolated  crypts  lie  on  the  other  side  of 
the  angular  line  in  the  ciliary  zone. 

When  the  pupillary  zone  has  an  especially  delicate  structure,  one 
can  at  times  make  out  the  sphincter  piipillae  as  a  whitish  band  of  about 
I  mm  in  width  shimmering  through  from  below,  immediately  adjoining 
the  pupil-border. 

As  a  rule,  the  ciliary  zone  shows  a  more  uniform  structure.  A  mark- 
ing made  by  fine,  radially  wavy  stripes  dominates  here;  the  blood-vessels 
of  the  iris  produce  this.  The  striation  is  seen  best  in  the  middle  of  the 
ciliary  zone.  In  the  neighborhood  of  the  angular  line  it  is  often  entirely 
covered  over  as  a  result  of  the  more  marked  development  of  the  anterior 
connective-tissue  layers,  and  this  part  is,  moreover,  also  the  thickest  part 
of  the  whole  iris.  The  inner  half  of  the  ciliary  zone  is  pretty  smooth; 
in  the  outer  half,  on  the  other  hand,  one  sees  the  so-called  contraction 
furrows  (/)  coursing  obliquely  over  the  meridional  striations;  these  are 
sharply  demarkated  circular  furrows,  running  concentric  with  the  ciliary 
border,  deepened  when  the  iris  is  narrowed,  and  almost  eradicated  when 
the  iris  is  broadened.  An  individual  contraction  furrow  extends  over  a 
greater  sector  of  the  iris,  but  never  over  the  whole  iris.  The  furrows  are 
fixed  structures  like  the  folds  of  the  skin  in  the  hollow  of  the  hand,  and 
are  to  be  made  out  even  when  obscure  (as  slight  deepenings  or  by  another 
color) . 

In  the  very  neighborhood  of  the  ciliary  border  crypts  again  appear 
(peripheral,  or  ciliary  crypts,  k).  They  are  much  smaller  and  shallower 
than  the  pupillary  ones  and  form  little  groups  or  rows  between  which 
the  larger  branches  of  the  iris  vessels  and  of  the  nerves  course.  So  the 
outermost  periphery  of  the  iris  (the  marginal  zone;  Fuchs,  67)  again  has 
a  net-form  structure  and  a  darker  coloring. 

This  zone  is  usually  not  visible  in  the  living  eye;  but  anatomic 
preparations  show  that  it  extends  to  the  ver\^  ciliary  border,  and  this  is 
thereby  broken  up  into  a  number  of  teeth  which  run  from  the  anterior 


128  ANATOMY  AND  HISTOLOGY  OF  THE  HUMAN  EYEBALL 

surface  of  the  ciliary  body  to  the  mcshwork  of  the  iris  angle.  When 
such  a  tooth  is  separated  wholly  from  the  substrata  and  bridges  over  the 
iris  angle  to  the  meshwork,  it  becomes  an  iris  process.  All  these  extensions 
of  the  iris  tissue  go  over  into  the  uveal  mcshwork  (cf.  p.  48). 

As  is  well  known,  the  color  of  the  anterior  surface  of  the  iris  varies 
greatly,  yet  in  general  two  types  can  be  distinguished,  the  light  (mostly 
blue-gray),  and  the  brown  iris.  The  two  types  are  not,  however,  sepa- 
rated alone  by  their  color,  but  they  differ  as  well  in  the  relief  of  the 
anterior  surface;  in  the  blue  iris  many  more  details  of  the  deeper  layers 
can  be  recognized;  the  radial  fibrillation  comes  out  much  more  markedly, 
the  crypts  are  more  numerous  and  larger;  the  contraction  furrows,  on  the 
other  hand,  are  only  to  be  seen  by  oblique  illumination.  The  brown  iris 
shows  a  grosser  relief;  the  ciliary  zone  is  smoother,  the  crypts  are  more 
sparse  and  smaller,  the  contraction  furrows,  on  the  other  hand,  are  more 
plain  and  are  visible  as  clear  lines  even  in  a  partly  obliterated  state.  When 
the  pupil  is  narrow,  one  can  also  see  radial  furrows  well  in  the  smooth 
part  of  the  ciliary  zone. 

The  posterior  surface  of  the  iris  (PI.  VIII,  2)  has  an  almost  uniformly 
dark  brown — almost  black — color  in  all  cases,  and  appears  smooth  to  the 
naked  eye.  On  moderate  magnification  and  a  strong  focal  light,  such  as 
sunlight,  a  fine  relief  of  radial  and  circular  folds  comes  out. 

The  radial  folds  form  two  systems  independent  of  one  another.  The 
one  consists  of  numerous  shallow  little  furrows  {rF),  goes  out  from  the 
pupil-border,  and  gradually  loses  itself  about  i  mm  away.  This  furrow 
system  (Schwalbe's  contraction  folds)  bends  about  the  pupil-border  onto 
the  anterior  surface  with  the  pigment  epithelium  and  produces  the 
crenations  of  the  pigment  seam  visible  from  in  front,  for  this  notching  is 
nothing  more  than  the  optical  cross-section  (the  profile)  of  the  furrow 
system. 

The  second  system  contains  fewer  but  deeper  furrows,  called  struc- 
tural furrows  (structural  folds,  Schwalbe),  because  they  are  present  in  the 
vessel  layer  as  well.  They  begin  about  i .  5  mm  from  the  pupil-border,  are 
narrow  and  deep  at  first,  then  broaden  out  and  flatten  down  toward  the 
ciliary  border  {SF).  Alongside  them  are  still  shorter,  shallower  furrows. 
The  number  of  well-developed  structural  furrows  is  significantly  less  than 
the  number  of  ciliary  valleys;  there  are  two  or  three  valleys  to  one  struc- 
tural furrow.  The  circular  furrows  {cF)  are  considerably  finer  than  the 
radial.  They  are  found  only  in  the  region  of  the  structural-furrow  sys- 
tem, and  cross  these  in  very  regular  arrangement. 

At  the  very  ciliary  border  the  pigment  ceases  somewhat  earlier  than 
does  the  vessel  layer,  for  the  torn  surface  (the  iris  root)  is  turned  somewhat 


THE  IRIS  129 

backward.  The  torn  edge  of  the  pigment  covering  also  shows  a  wavy 
margin,  because  the  ciliary  processes  are  to  some  extent  grown  to  the 
back  surface  of  the  iris. 

The  preparation  of  sections  in  the  three  main  directions  is  demanded 
for  an  accurate  understanding  of  the  anatomy  and  histology  of  the  iris. 
These  directions  and  the  pictures  corresponding  to  them  are : 

(i)  The  meridional  or  radial  section  (Taf.  I),  i.e.,  the  section  in  the 
direction  of  the  iris  radius  and  perpendicular  to  the  surface  of  the  iris, 
shows  a  very  irregular  limitation  in  front.  The  various  crypts  are 
recognized  as  interruptions  of  the  most  anterior  layer  of  the  tissue  (^,,  k,), 
the  contraction  furrows  as  sharp  angular  indentations  of  this  layer  (/) ; 
the  maximum  thickness  in  the  region  or  neighborhood  of  the  angular 
line,  the  gradual  slope  of  the  anterior  surface  toward  the  pretty  sharp 
border  of  the  pupil,  the  bending  of  the  posterior  pigment  covering,  and 
the  ending  of  this  layer  are  seen  at  the  pupil-border.  One  sees  the 
sphincter  papillae  stretching  out  from  this  border  into  the  stroma  of  the 
iris,  the  firm  connective  tissue  which  supports  it  from  behind  and  unites 
it  with  the  posterior  surface  of  the  iris;  one  recognizes  the  more  uniform 
thickness  of  the  ciliary  zone  and  the  thinning  toward  the  root  of  the  iris. 

When  the  section  falls  between  two  structural  furrows,  the  posterior 
margin  of  the  iris,  as  a  whole,  appears  straight  as  a  line;  in  other  cases, 
it  shows  a  step  slightly  ciliaryward  to  the  peripheral  margin  of  the  spJiinctcr 
pupillae.  In  the  ciliary  portion  the  circular  furrows  appear  as  regular 
crenations  of  the  pigment  covering,  while  this  appears  smooth  toward 
the  pupil-border  because  the  furrows  lying  here  course  in  the  direction 
of  the  section. 

(2)  The  section  at  right  angles  to  the  radius  may  be  called  the  trans- 
verse section.  Yet  such  a  section  is  strictly  a  transverse  one  at  only  a 
single  point  owing  to  the  radial  course  of  most  of  the  structural  elements. 
The  farther  one  goes  from  this  point  the  more  oblique  are  the  elements 
encountered,  and  the  less  clear  is  the  picture.  Naturally,  the  picture 
varies  according  to  the  part  of  the  iris  through  which  the  section  goes ;  the 
lateral  portion  of  such  a  section  always  contains  the  more  ciliaryward 
lying  portions. 

In  general,  the  transverse  section  shows  a  smoother  course  along  the 
anterior  border  of  the  iris;  on  the  other  hand,  the  structural  furrows  on 
the  back  surface  come  out  very  plainly;  the  posterior  limitation  is  a 
wavy  line. 

(3)  The  surface-section,  i.e.,  the  section  parallel  to  the  surface  e.xpanse 
of  the  iris,  presents  apparently  the  least  instructive  picture,  partly  because 
the  individual  layers  of  the  iris  are  much  too  lacking  in  evenness  for  the 


130  ANATOMY  AND  HISTOLOGY  OF  THE  HUMAX  EYEBALL 

section  not  to  vary  into  either  the  next  higiier  or  lower  lajer.     Neverthe- 
less, the  surface-section  is  of  great  significance  for  the  histology  of  the  iris. 


Like  the  ciliary  body,  the  iris  contains  portions  of  the  tunica  vasculosa, 
the  stratum  pigmcnti,  and  the  tunica  interna  (cf.  p.  14);  yet  there  is  such 
an  intergrowth  of  these  coats  in  the  territory  of  the  iris  that  a  division 
of  the  layers  of  the  iris  on  this  basis  cannot  be  carried  out. 

The  arrangement  of  the  layers  of  the  iris  is  regular  only  in  the  middle 
part  of  the  ciliary  zone;  therefore  the  following  survey  holds  true  only 
for  this  portion,  and  the  beginner  will  do  well  to  take  up  the  study  of  the 
microscopic  anatomy  of  the  iris  with  this  portion. 

The  part  of  the  iris  belonging  to  the  tunica  vasculosa  is  called  the  pars 
uvealis  iridis.  It  consists  mainly  of  a  continuation  of  the  vessel  layer  of 
the  ciliary  body — the  vessel  layer  of  the  iris  (PL  VIII,  3,  G).  On  the 
anterior  surface,  this  layer  is  characterized  by  the  disappearance  of  the 
vessels  and  the  preponderance  of  the  cells  of  the  anterior  border  layer 
{vG);  on  the  side  of  the  anterior  chamber  this  is  closed  off  by  an  endo- 
thelium which  is  connected  with  the  endothelium  of  the  trabeculae  of 
the  iris  angle  and  thereby  with  that  of  the  posterior  surface  of  the  cornea. 

The  layers  belonging  to  the  stratum  pigmenti  and  the  tunica  interna, 
therefore,  the  layers  of  ectodermal  origin,  have  many  times  been  placed 
together  under  the  name  pars  retinalis  iridis,  or  pars  iridica  retinae. 
Here,  to  prevent  misunderstanding,  the\-  will  be  designated  as  the  ecto- 
dermal layers  of  the  posterior  surface  of  the  iris.  (Compare  what  was 
said  on  p.  13  concerning  the  conception  of  the  retina.) 

The  continuation  of  the  stratum  pigmenti  grows  into  an  epithelium 
muscle,  the  musculus  dilatator  pupillae  (Di)  in  the  region  of  the  iris; 
it  appears  to  di\ide  again  into  two  layers,  namely,  a  non-nucleated, 
membrane-like  layer  in  front  (posterior  border  lamella)  and  a  layer  of 
nucleated,  pigmented,  spindle-form  elements  behind  it  (layer  of  the 
pigmented  spindle  cells).  The  continuation  of  the  tunica  interna  is  a 
simple  epithelium  of  densely  pigmented  cells,  the  pigment  epithelium  of 
the  iris  (P). 

So,  from  in  front  backward  the  iris  may  be  divided  into  the  following 

layers: 

(  I.  Endothelium 

a)  Pars  uvealis  iridis  I  2.  Anterior  border  layer 

^'  ^  (a)  Posterior  border  lamella 

b)  Ectodermal  layers  of  the  \  4.  Dilatator  pupillae  -j  ^^  ^-^^^^^^^  ,pi„dig  .^Hs 
back  surface  of  the  iris               j  5.  Pigment  epithelium 

The  sphincter  pupillae  and  the  clump  cells  are  also,  indeed,  of  ectoder- 
mal origin,  although  anatomically  they  are  imbedded  in  the  vessel  layer 
of  the  iris  and  will  be  considered  after  the  vessel  layer. 


THE  IRIS  131 

a)    The    Uveal  Portion  of  the  Iris  {Pars  uvcalis  iridis) 

I.       THE    ENDOTHELIUM   OF   THE   IRIS 

In  general,  it  is  true  that  it  has  been  settled  that  the  anterior  surface  of  the  iris  is 
covered  by  an  endothelium,  but  the  descriptions  of  this  layer  differ  greatly.  Aside 
from  the  possibility  of  an  Imbricated  covering  of  endothelial  cells  (Arnold,  8),  which 
in  and  of  itself  is  very  improbable,  the  endothelium  is  sometimes  described  as  similar 
to  that  of  Descemet's  membrane  although  made  up  of  smaller  cells  (Iwanoff,  115), 
sometimes  as  a  weakly  granular  membrane  possessed  of  a  moderate  number  of  nuclei 
which  can  be  isolated  by  maceration  in  Mueller's  fluid  (von  Michel,  155).  Moreover, 
it  is  demonstrable  only  in  young  individuals,  and,  in  general,  with  much  greater  difficulty 
in  the  human  than  in  animals  (Koganei,  iig). 

The  endothelium  of  the  iris  is,  indeed,  an  object  very  difficult  of  his- 
tologic demonstration.  The  endothelium  practically  cannot  be  demon- 
strated in  sections,  because  the  anterior  border  layer  with  its  numerous 
cells  and  nuclei  lies  immediately  under  it.  Only  when  these  cells  are  very 
densely  pigmented,  as  in  dark  brown  irides,  can  one  recognize  nuclei  on 
the  surface  here  and  there;  these  are  surrounded  by  an  unpigmented 
protoplasm.  Such  cells  are  probably  to  be  looked  upon  as  endothelial 
cells,  but  one  must  not  confound  them  with  wandering  cells;  they  are 
found  much  too  sparsely  to  give  the  impression  of  a  connected  cell-layer. 

The  classical  method  for  the  demonstration  of  cell  borders  (by  silver 
nitrate)  brings  out  a  very  complicated  system  of  lines  in  the  normal  iris 
of  older  people  (PL  VIII,  4) ;  here,  larger  cell-like  fields  alternate  with 
numerous  very  small  slender  and  often  poorly  demarkated  fields.  One 
does  not  get  the  impression  that  the  borders  form  a  single  layer  of  flat 
cells.  Other  elements  than  the  endothelial  cells  of  the  anterior  border 
layer  are  possibly  drawn  into  the  formation  of  particular  portions  of  the 
anterior  iris  surface. 

2.       THE    ANTERIOR   BORDER   L.WER 
(The  anterior  stroma  leaf;  Krueckniann,  125) 

This  is  only  a  modification  of  the  iris  stroma,  yet  it  differs  from  this 
in  its  greater  density,  and  is  of  special  significance  because  it  has  to  do 
with  the  color  of  the  iris. 

It  is  principally  made  up  of  cells  between  which  there  are  only  a  very 
few  collagenous  fibrillae  and  numerous  nerve-endings,  but  no  blood-vessels. 

The  cells  are  chromatophores,  like  the  stroma  cells,  and  usually  possess 
only  two  or  three  processes  (sometimes  more,  sometimes  less).  These 
processes  are  often  arranged  in  little  bundles,  especially  in  the  zone  of  the 
contraction  furrows,  so  that  a  porous  appearance  is  produced  in  the 
anterior  border  layer  (PL  VIII,  5).  A  net-form  union  probably  exists 
between   them.     Through  repeated   superimposition  of    such  cells  and 


132  ANATOMY  AND  HISTOLOGY  OF  THE  HUMAN  EYEBALL 

through  the  crossing  of  the  processes  there  arises  a  very  thick  plexus,  very 
dilTicult  of  solution,  and  which  gradually  goes  over  into  the  iris  stroma 
behind  (inward)  by  a  gradual  spacing  up  of  the  cell  framework  and  an 
increasing  preponderance  of  collagenous  interstitial  substance. 

In  the  middle  portions  of  the  iris,  where  it  is  cjuite  smooth,  the  plexus 
is  very  uniformly  developed  in  all  directions;  at  the  border  of  the  crypts, 
however,  the  i)rocesses  are  arranged  more  parallel  to  this  border. 

The  pigment,  the  mass  of  which  varies  a  great  deal,  individually,  con- 
sists of  finer  and  grosser  granules.  Concerning  the  endings  of  the  nerve- 
fibers,  which  are  demonstrable  only  by  the  methyl-blue  method,  see  below 

(P-  134)- 

The  thickness  of  the  anterior  border  layer  varies  a  great  deal  in  the 
different  portions  of  the  iris.  It  fails  entirely  at  the  entrances  of  the 
crypts;  only  in  the  places  where  a  pupillary  crypt  is  large  and  extends 
obliquely  into  the  stroma  has  the  wall  of  the  crypt  facing  forward  a 
thick  border  layer.  Furthermore  it  is  very  much  thinner  on  the  floor  of 
the  contraction  furrows  than  in  the  neighborhood;  on  this  account  the 
contraction  furrow  can  never  be  completely  obliterated.  It  is  thickest 
on  the  border  of  the  pupillary  and  the  ciliary  zones  and  thereby  obscures 
the  structure  of  the  vessel  layer. 

The  anterior  border  layer  gives  the  color  to  the  iris,  not  alone  through 
its  pigment  content  but  also  its  density;  blue  irides  have  a  delicate  border 
layer  and  almost  unpigmented  cells;  brown  irides  have  a  thick  border 
layer  and  very  heavily  pigmented  cells. 

A  complete  absence  of  pigment  in  the  border  layer  and  in  the  stroma 
probably  occurs  only  in  the  newborn  and  in  very  young  children.  These, 
therefore,  at  times  actually  have  blue  eyes,  for  the  blue  color  is  only  due 
to  the  fact  that  a  clouded  but  colorless  medium  (border  layer  and  stroma) 
lies  in  front  of  a  dark  background  (pigment  epithelium).  A  complete 
absence  of  pigment  scarcely  occurs  in  the  adult,  and  the  blue  of  such  eyes 
appears  dulled,  runs  into  gray,  or  more  rarely  into  a  greenish  hue. 

This  difference  also  comes  out  when  the  color  in  different  sectors  of  the  iris  varies. 
A  different  color  in  the  two  eyes  of  the  same  individual  (one  eye  blue,  the  other  brown), 
heterochromia  iridis,  can  up  to  a  certain  extent  be  physiologic  and  is  due  to  the  above- 
reported  state  of  the  border  layer;  it  is,  however,  also  often  pathologic,  especially  when 
the  individual  is,  in  general,  of  the  brunette  type.  In  these  cases  the  blue  eyes  very 
eas'ly  get  iridocyclitis  and  cataract,  and  it  has  been  shown  anatomically  that  in  such 
cases  the  blue  color  of  the  iris  is  only  the  expression  of  a  chronic  disease  (atrophy  of 
the  iris;  Fuchs,  68). 

Albinism  is  a  developmental  anomaly  and,  therefore,  does  not  come  into  considera- 
tion here.  In  passing,  it  may  only  be  mentioned  that  it  is  due  to  the  lack  of  color  of 
the  pigment  epithelium.     The  stroma  pigment  does  not  need  to  fail  entirely  in  such  eyes. 


THE  IRIS  133 

3.      THE    VESSEL   LAYER 

This  forms  the  main  mass  of  the  iris.  It  contains  the  numerous  larger 
blood-vessels  and  nerve-plexuses,  held  together  by  a  very  loose  delicate 
stroma. 

The  blood-vessels  of  the  iris  enter  the  iris  root  between  the  peripheric 
crypts  (PI.  VIII,  i)  in  larger  bundles  and  branch  into  finer  branches  as 
soon  as  they  have  passed  the  zone  of  these  crypts ;  these  pass  through  the 
ciliary  zone  in  a  radial  direction  in  several  layers.  They  give  the  ciliary 
zone  its  meridional  striation.  The  vessels  usually  show  a  corkscrew-like 
winding,  for  only  in  this  way  can  they  adapt  themselves  to  the  changing 
states  of  contraction  in  the  iris  tissue.  The  narrower  the  pupil,  the 
broader  the  iris,  and  the  blood-vessels  must  be  just  so  much  the  more 
stretched  out,  and  then  the  meridional  section  shows  longitudinal  sections 
of  the  vessels  almost  exclusively.  On  the  other  hand,  the  wider  the  pupil, 
the  narrower  the  iris,  and  just  so  much  closer  are  the  windings  of  the 
vessels  upon  the  meridional  section ;  each  vessel  is  then  broken  up  into  a 
series  of  cross-sections. 

In  the  pupillary  portion  this  course  is  somewhat  changed,  partly  in  the 
formation  of  anastomoses  (smaller  circle),  partly  in  the  supply  of  the 
sphincter  pupillae;  many  circular  vessels  are  also  to  be  found  here. 

All  of  the  vessels  of  the  iris  are  characterized  by  a  thick  adventitia; 
this  consists  of  a  finely  fibrillated  and  therefore  almost  hyalin-appearing 
collagenous  tissue,  the  thickness  of  which  often  exceeds  the  diameter  of 
the  vessel  lumen.  The  arteries  possess  a  thin  muscularis  and  a  very 
weak  intima,  which  many  times  does  not  stain  by  orcein;  the  veins 
have  perivascular  sheaths  (perithelium)  bordering  immediately  upon  the 
endothelium. 

According  to  Leber  (138),  the  arteries  of  the  iris  give  off  a  pretty  wide- 
meshed  capillary  net  in  the  ciliary  zone  going  through  the  entire  thickness 
of  the  vessel  layer;  a  special  capillary  net  is  found  neither  on  the  anterior 
nor  on  the  posterior  surface.  The  capillary  net  first  becomes  narrower  at 
the  sphincter  pupillae.  The  last  ends  of  the  arteries  bow  over  into  the 
veins. 

The  nerves  of  the  iris  likewise  advance  through  the  root  of  the  iris 
in  larger  trunks  and  then  build  a  plexus  in  front  of  the  larger  vessels 
(Pause;  169);  often  these  branches  consist  of  only  a  few  fibers,  which, 
aside  from  the  meridional  course,  also  run  obliquely  and  crosswise. 
But  the  nerves  can  only  be  followed  over  longer  stretches  and,  therefore, 
identified  with  certainty  in  teased  preparations.  Only  in  such  preparations 
does  one  get  an  idea  of  the  richness  of  the  iris  in  nerves.  Nerves  are 
practically  not  to  be  recognized  in  cut  sections. 


134  ANATOMY  AND  HISTOLOGY  OF  THE  HUMAN  KYKBALL 

The  nerves  in  the  larger  trunks  and  branches  are  partly  medullated, 
partly  non-medullated.  The  individual  branches  possess  a  thick  con- 
nective-tissue hull  (neurilemma),  which  in  this  respect  is  like  the  adventitia 
of  the  blood-vessels.  Moreover,  the  relations  of  the  neighboring  stroma 
cells  to  the  nerves  is  the  same  as  in  the  case  of  the  blood-vessels. 

The  nerve-fibers  end  partly  in  the  stroma  (sensory  fibers),  partly  in 
the  vessels  (sympathetic  fibers),  partly  in  the  sphincter  pupillae,  partly 
in  the  dilatator  (motor  fibers).  Yet  there  is  only  very  little  known 
concerning  the  nature  of  the  ending  in  man,  for  the  study  encounters 
enormous  difficulties  as  a  whole  in  the  human  iris  on  account  of  its 
thickness  and  richness  in  pigment. 

According  to  Meyer  (153),  a  sensory  net  l}dng  immediately  under  the  endothelium 
goes  out  of  the  wide-meshed  plexus  of  the  iris  nerves  in  rabbits ;  furthermore,  the  motor 
fibers  for  the  sphincter,  which  form  a  net  made  up  of  long-strung-out  meshes  between 
the  muscle-fibers  and  the  vasomotor  fibers  of  the  iris  vessels  and  form  two  plexuses 
in  each  artery,  one  in  the  adventitia,  the  other  in  the  muscularis,  are  also  given  off  by 
this  same  plexus.  Retzius  (179)  also  demonstrated  numerous  nerve-fibers  in  the  posterior 
border  lamella.  Muench  (162),  finally,  postulates  a  union  of  the  nerve-fibers  and  the 
stroma  cells  (cf.  p.  135). 

Opinions  concerning  the  presence  of  ganglion  cells  in  the  iris  are 
divided.  Meyer  depicts  two  cells  in  a  human  iris  which  look  exactly 
like  ganglion  cells,  yet  no  union  with  nerve-fibers  can  be  made  out;  I 
myself,  have  upon  one  occasion,  but  only  upon  one  occasion,  seen  an 
uncjuestionable  ganglion  cell  in  the  iris.  The  views  of  Muench  concerning 
this  matter  will  be  given  consideration  later  on  (p.  135). 

The  spaces  between  the  blood-vessels  and  the  nerves  are  filled  out  by 
the  iris  stroma  proper.  This  is  an  extremely  delicate,  loose,  collagenous 
tissue  containing  the  pigmented  stroma  cells  (chromatophores),  non- 
pigmented  stroma  cells,  clump  cells,  and,  finally,  sparse  wandering  cells. 

The  collagenous  intervening  tissue  (PL  VII  6,  b)  consists  of  very 
delicate  and  discrete  fibrillae  of  such  fineness  that  only  intensive  staining, 
e.g.,  Mallory's  hematoxylin,  give  a  clear  picture.  The  fibrillae  are  not 
arranged  in  bundles.  Their  course  in  the  middle  parts  of  the  iris  and  in  the 
depths  of  the  vessel  layer  is  meridional.  Toward  the  ciliary  border  they 
go  over  into  a  fiber  plexus  and  also  in  the  pupillary  zone  they  change  the 
course  of  their  fibers.  The  intermediary  substance  is  most  markedly 
developed  behind  the  sphincter  pupillae  (cf.  p.  137). 

Elastic  fibers,  i.e.,  fibers  which  stain  with  orcein,  fail  almost  entirely 
in  the  iris;  a  few  isolated  fibers  of  this  nature  are  found,  but  only  in  the 
peripheral  portions;  these  are  apparently  radiations  of  the  elastic  frame- 
work at  the  insertion  of   the   ciliarv   muscle   into   the   scleral    roll.     In 


THE  IRIS  135 

connection  with  closely  compressed  collagenous  fibrillae  they  course  into 
the  iris  in  a  meridional  direction. 

According  to  de  Lieto  Vollaro  (143),  elastic  fibers  are  also  found  in  the 
tissue  behind  the  sphincter,  yet  so  far  I  have  not  been  able  to  see  these 
fibers.  In  any  case,  the  elastic  tissue  of  the  iris  is  very  much  less  in 
amount  than  in  the  other  portions  of  the  uveal  tract.  This  is  all  the  more 
striking  because  when  wounded  the  iris  shows  an  especial  tendency  to  gap. 

The  chromatophores  are  grouped  principally  about  the  vessels  and 
the  nerves — the  adventitia  (and  especially  the  neurilemma)  of  which 
they  invest  with  their  processes.  The  interstices  proper  between  the 
vessels  are  permeated  by  a  very  loose  framework  of  stroma  cells.  This 
framework  is  somewhat  thicker  in  the  pupillary  zone,  especially  in  the 
neighborhood  of  the  sphincter  and  at  the  ciliary  border  of  the  dilatator 
lamella. 

Each  chromatophore  shows  a  small  oval  body,  which  stains  well, 
and  an  oval  nucleus  not  surrounded  by  pigment ;  the  processes  are  slender 
and  long  (up  to  100  mu)  and  few  in  number.  They  unite  with  those  of 
their  neighbors  into  a  plexus.  The  pigment  is  finely  granular,  and  for 
the  most  part  much  paler  than  in  the  chromatophores  of  the  chorioidea. 

With  respect  to  the  direction  of  the  cell-processes  one  can  only  say 
that  no  particular  direction  rules  in  the  anterior  part  of  the  vessel  layer, 
and  that  a  meridional  course  comes  out  in  the  depths.  Especially  in  the 
very  deepest  stroma  layer  immediately  in  front  of  the  dilatator  one  en- 
counters very  much  elongated,  bipolar  chromatophores  with  a  meridional 
direction.  Finally,  one  sees  divergent  stroma  cells  radiating  out  from  the 
thickened  places  in  the  dilatator  lamella  at  the  ciliary  border  toward 
the  anterior  iris  surface. 

Aside  from  the  chromatophores,  non-pigmented  stroma  cells  are  also 
found.  These  likewise  possess  processes,  yet  these  are  much  more  deli- 
cate and  fine.  This  kind  of  cells  is  held  to  be  nerve-cells  b)^  Muench 
(162). 

According  to  this  author,  there  are  no  transitions  between  the  two 
kinds  of  stroma  cells.  In  man  the  distinction  is  often  difficult — when  the 
chromatophores  are  little  pigmented — yet  their  processes  are  larger  than 
are  those  of  the  unpigmented  stroma  cells.  In  the  much  more  hea\'ily 
pigmented  iris  of  apes  the  difference  is  much  more  plain. 

Muench  (160)  held,  furthermore,  as  reported  above  (p.  50),  that  the 
chromatophores  are  muscle  cells  and  states  that  they  are  united  with 
nerve-fibers;  the  latter  form  a  network,  the  nodal  points  of  which  are  the 
above-described  unpigmented  stroma  cells,  and  either  press  into  the  body 
of  the  chromatophore  or  are  attached  to  it  by  means  of  conical  insertions. 


136  ANATOMY  AND  HISTOLOCJV  OF  THP:  HUMAN  EY1:BALL 

Aside  from  the  chromatophores,  characterized  by  their  processes, 
larger  pigment  cells  are  found  in  the  neighborhood  of  the  sphincter 
pupUlae  and  occasionally  also  in  the  neighborhood  of  the  ciliary  border. 
They  are  without  processes  and  therefore  rounded;  their  pigment  is  made 
up  of  large,  round,  and  very  dark  granules.  They  have  long  been  known 
by  the  name  clump  cells  (Koganei,  119)  (PI.  VIII,  3,  10,  K).  Their 
true  nature  has,  however,  only  recently  been  made  clear  by  Elschnig 
and  Lauber  (55);  they  are  cells  displaced  out  of  the  ectodermal  layers 
of  the  posterior  surface.  This  is  shown  not  only  by  the  structure  of  the 
pigment  but  also  by  its  density  (it  covers  the  cell-nucleus),  and  finally  by 
the  circumstance  that  these  cells  are  just  as  intensely  pigmented  in  blue 
irides  as  in  brown.  These  cells  have  renounced  their  epithelial  nature  in 
only  one  matter;  they  have  lost  the  tendency  to  form  closed  bands  and 
lie  wholly  isolated  in  the  stroma.  These  cells  are  also  to  be  found  in  the 
sphincter  pupillae  and  in  the  connective  tissue  behind  this  muscle.  The 
most  conclusive  proof  of  their  nature  is,  however,  found  in  those  cases 
in  which  there  is  a  defect  in  the  sphincter  and  the  cells  radiate  out  from 
the  posterior  surface  through  this  into  the  iris  stroma  (cf.  loc.  cit.,  55;  PI. 
XIX,  Fig.  i). 

The  number  of  wandering  cells  is  very  small,  at  least  in  the  normal 
iris:  these  are  small,  round,  sharply  contoured  cells  with  homogeneous 
or  weakly  granular  protoplasm  and  a  small,  heavily  stained,  round  or 
lobulated  nucleus.  Other  forms  are  probably  pathologic,  even  when  no 
other  diseased  changes  are  found  in  the  iris,  for  the  iris  is  very  easily 
affected  in  diseases  of  the  other  tissues  of  the  eye  or  body. 

In  general,  the  iris  stroma  is  set  off  from  the  anterior  chamber  by  the 
anterior  border  layer  and  the  endothelium.  This  delimitation,  however, 
fails  in  the  crypts ;  the  crypts  are  places  in  which  the  aqueous  bathes  the 
stroma  of  the  vessel  layer.  On  histologic  examination,  the  peripheral 
crypts  appear  as  simple  defects  of  the  anterior  border  layer  and  of  the 
endothelium  and  in  this  way  a  deeper  layer  of  the  stroma  is  exposed;  the 
form  of  these  crypts  is,  therefore,  that  of  pit-like  deepenings. 

The  larger  pupillary  crypts  are  hollowed  out  of  the  stroma  of  the 
iris,  on  the  other  hand;  they  often  stretch  out  toward  the  periphery  in 
such  a  way  that  the  peripheral  border  of  the  crypt  appears  undermined. 
Many  times  the  entrance  to  such  a  crj'pt  is  bridged  over  by  free  trabeculae 
(PI.  I  shows  a  cross-section  of  a  trabecula  at  k^).  Viewed  from  the  front, 
the  floor  of  such  a  crypt  is  not  entirely  obscured  by  a  border  layer  and  this 
is  only  much  weaker  developed  here  than  on  the  anterior  surface  of  the 
iris.     According  to  Fuchs  (67),  who  first  studied  the  histologic  relations 


THE  IRIS  137 

in  these  crypts  more  accurately,  the  endothelial  covering  is  interrupted 
at  the  crypts  and  the  spaces  in  the  tissue  of  the  iris  stroma  communicate 
freely  with  the  anterior  chamber.  At  the  same  time  it  is  not  possible 
to  inject  the  tissue  spaces  of  the  iris  from  the  chamber. 

The  vessel  layer  undergoes  a  special  modification  in  the  pupillary  zone 
of  the  iris  by  the  interposition  of  the  structure  which  contracts  the  pupil 
{m.  sphincter  pupillae).  From  a  developmental  standpoint  this  muscle 
belongs  to  the  ectodermal  layers  of  the  iris,  it  is  true,  yet  throughout 
its  development  it  is  so  completely  imbedded  in  the  vessel  layer  that  it 
can  only  be  treated  in  connection  with  the  vessel  layer  in  the  anatomic 
description  of  the  iris. 

The  sphincter  pupillae  (PI.  VIII,  3,  Sph)  forms  an  annular  band  some 
0.9  mm  broad,  of  which  the  inner  (pupillary)  border  is  entirely  closed 
off  by  the  border  of  the  pigment  epithelium  {Ps).  I  doubt  very  much 
whether,  in  general,  a  true  connective  tissue  limitation  is  present. 

The  sphincter  is  made  up  of  bundles  which  cross  each  other  at  very 
narrow  angles  and  form  a  framework  similar  to  that  in  the  ciliary  muscle. 
The  direction  of  the  bundles  is  purely  circular  on  the  surface  of  the  muscle 
(concentric  with  the  pupil-border)  and  parallel  with  the  surface  of  the 
pupil.  The  bundles  are  thick,  the  intervening  tissue  sparse.  Toward 
the  back  surface  the  framework  is  somewhat  more  loose,  the  bundles  more 
slender,  the  intervening  tissue  richer,  the  variations  from  the  strictly 
circular  course  more  marked,  and,  moreover,  bundles  are  found  which 
course  obliquely  toward  the  dilatator  lamella  or  the  pigment  epithelium 
(PI.  VIII,  10).  The  intervening  tissue  of  this  part  and  the  connective 
tissue  lying  behind  the  sphincter  is  especially  rich  in  collagenous  fibrillae 
and,  therefore,  shows  a  much  denser  structure  than  the  rest  of  the  iris 
stroma;  according  to  de  Lieto  Vollaro,  elastic  fibers  are  also  found 
in  it. 

As  a  result,  the  sphincter  shows  a  well-marked  limitation  in  front 
on  meridional  sections  but  not  behind  toward  the  supporting  connective 
tissue.  Both  layers  thicken  gradually  from  the  pupil-border  toward  the 
ciliary  border  of  the  sphincter  and  attain  there  a  thickness  of  o.i  to 
0.17  mm. 

The  bundles  of  the  sphincter  consist  of  smooth  muscle-fibers.  Possibly 
they  have  a  shorter,  more  oval,  therefore,  less  rod-form  nucleus,  but 
otherwise  they  agree  completely  with  those  of  the  usual  form.  Their 
protoplasm  stains  pretty  heavily  with  eosin,  ammonia-carmin,  indigo- 
carmin,  takes  on  a  yellowish  to  orange-yellow  nuance  by  Van  Gieson's 
stain,  and  shows  sharp,  plain  contours,  for  each  fiber  is  surrounded  by 


138  ANATOMY  AND  HISTOLOGY  OF  THE  HUMAN  EYEBALL 

a  delicate  sheath  of  connective  tissue;   myoglia  fibrillac  are  not  found  in 
the  sphincter  fibers  (Forsmark,  59). 

The  connective  tissue  behind  the  muscle  ajiparently  serves  to  support  it  and  effects 
a  firmer  union  between  the  dilatator  piipillac  and  sphincter,  on  the  one  side,  and  with  the 
pupil-border,  on  the  other. 

Concerning  the  union  between  the  sphincter  and  the  dilatator  piipillac, 
see  the  latter. 

b)    Ectodermal  Layers  of  the  Back  Surface  of  the  Iris 
4.   OUTER  leaf:  Dilatator  piipillac 

The  recent  embryologic  investigations  of  Grynfeltt  (80),  in  animals, 
of  Heerfordt  (88),  von  Szili,  Jr.  (216),  and  Herzog  (95),  in  man,  have  first 
made  clear  the  nature  and  the  origin  of  the  dilatator  piipillae  to  us: 
the  dilatator  pupillac,  like  the  sphincter,  is  an  epithelial  muscle,  i.e., 
its  fibers  develop  out  of  epithelial  cells,  and,  indeed,  out  of  those 
of  the  outer  layer  of  the  optic  vesicle.  But  while  a  complete  transition  of 
the  epithelial  cell  into  a  muscle  cell  occurs  in  the  sphincter  pupillae, 
this  takes  place  in  only  a  part  of  the  cell  (its  basis)  in  the  dilatator 
pupillae;  the  head  of  the  cell  has  an  epithelial  character  and  maintains 
pigmentation. 

Therefore,  when  typically  developed,  the  dilatator  element  appears 
as  a  spindle-form  cell  with  an  oval  nucleus  and  a  moderately  pigmented 
protoplasm,  extended  at  each  end  into  an  unpigmented  fiber-like  process 
(PL  VIII,  7).  Since  these  processes  correspond  to  the  cell-basis,  they 
lie  at  another  level  (farther  forward)  than  the  nucleated  head  of  the  cell, 
and  in  a  cursory  look  at  the  section  the  dilatator  pupillae,  therefore, 
appears  to  be  made  up  of  two  layers :  one,  a  non-nucleated  membranous 
layer  in  front  (posterior  border  lamella  or  border  membrane  of  Fuchs, 
Bruch's  or  Henle's  membrane)  and  the  other,  a  layer  of  nucleated  pig- 
mented spindle  cells  (anterior  pigment  layer  of  Fuchs,  anterior  epithelium 
of  Gruenhagen  and  others). 

This  description  contains  much  which  is  not  at  once  to  be  seen  in  the 
preparation  but  can  only  be  deduced  from  the  whole  outcome  of 
the  investigation.  We  have,  indeed,  made  considerable  progress  in  the 
recognition  of  the  anatomic  make-up  of  the  posterior  layers  of  the  iris 
by  means  of  the  various  methods  of  bleaching  the  section,  yet  owing  to 
the  fact  that  it  is  scarcely  ever  possible  to  isolate  the  dilatator  element 
completely  and  intact,  phantasy  stiU  plays  a  certain  part.  In  order  now 
to  meet  the  criticism  that  I  have  been  guided  more  by  phantasy  than 
by  the  actual  circumstances  I  add  a  purely  anatomic  description  of  the 
dilatator  pupillae. 


THE  IRIS  139 

The  posterior  border  lamella  (Pi.  \T1I,  8,  9,  liG)  is  a  layer  of 
some  4  mu  thickness,  which  does  not  always  show  a  sharp  delimitation, 
behind  at  least.  On  meridional  section  (PI.  VIII,  8)  it  appears  almost 
homogeneous  or  indistinctly  striated  longitudinally;  on  surface-section  it 
shows  a  plain  though  fine,  straight  meridional  striation;  on  transverse 
section  (PL  VIII,  9)  it  breaks  up  in  very  small,  rounded  or  angular  fields 
united  into  little  groups,  but  forming  a  continuous  layer  as  a  whole; 
it  shows  the  staining  reaction  of  protoplasm:  ammonia-carmin  stains  it 
especially  densely,  Van  Gieson  stains  it  orange-yellow;  orcein-  and  resorcin- 
acid  fuchsin  stains  are  negative.  From  all  of  this  it  follows  that  this 
layer  is  made  up  of  straight  protoplasmic  fibers  coursing  meridionally 
and  forming  a  prett}-  uniform  layer  not  plainly  separated  into  bundles. 
When  typically  developed,  cell-nuclei  are  not  found  in  this  layer. 

Aside  from  the  above-described  protoplasmic  tibers,  the  posterior  border  lamella 
contains  a  second  kind  of  fibers,  especially  characterized  by  the  fact  that  they  stain 
intensely  and  electively  with  iron-hematoxylin.  These  fibers  were  first  observed  by 
Widmark  (237),  later  more  accurately  described  by  Forsmark  (59)  and  at  the  same  time 
recognized  as  identical  with  the  constituent  portion  of  the  smooth  muscle  tissue,  called 
myoglia  by  Benda  (ig). 

The  myoglia  fibers,  according  to  Benda,  lie  intra-  and  extracellular;  the  same 
relationship,  therefore,  exists  as  in  the  cells  and  fibers  of  the  neuroglia.  So  far  as  can 
be  recognized  in  the  drawings  of  Widmark  and  Forsmark  at  hand,  the  myoglia  fibers  of 
the  dilatator  lie  almost  exclusively  between  the  protoplasmic  fibers  or  on  their  surface; 
in  part,  too,  they  press  down  in  between  the  pigmented  spindle  cells. 

Upon  meridional  section  the  layer  of  pigmented  spindle  cells  (PI. 
VIII,  8,  9,  sp)  is  only  indistinctly  set  off  against  the  posterior  border 
lamella,  toward  the  pigment  epithelium,  however,  very  sharply.  The 
borders  between  the  cells  can  scarcely  be  made  out  at  all.  These  borders 
are  seen  best  in  depigmented  preparations  as  fine  lines  running  obliquely 
toward  the  posterior  border  lamella  and  going  over  into  the  longitudinal 
strialions  of  it.  Possibly,  however,  these  lines  are  caused  by  the  myoglia 
fibers.  The  nuclei  of  the  spindle  cells  are  long  and  placed  parallel  to  the 
border  lamella.  The  protoplasm  contains  a  moderate  amount  of  pigment 
not  covering  the  nucleus;  its  granules  are  throughout  of  the  same  size, 
form,  and  color  as  those  of  the  pigment  epithelium.  The  thickness  of  the 
entire  layer  is  8  mu. 

The  spindle-form  suggested  in  the  meridional  section  comes  out  with 
entire  clearness  in  the  surface  view  (PI.  VIII,  7).  Here  the  cells  show  a 
regular  spindle  form,  they  are  some  7  mu  broad,  and  approach  60  mu  in 
length;  the  nucleus  shows  the  same  form  as  on  meridional  section,  i.e.,  a 
breadth  of  4  to  6  mu  and  a  length  of  something  like  14  mu.  The  axes  of 
the  spindle  cells  and  likewise  those  of  the  nucleus  have  a  strictly  meridional 


140  ANATOMY  AND  HISTOLOOV  OF    I'HK  HUMAN   K\KBALL 

dircclion."  The  pigment  is  located  principally  in  the  tapering  portion 
of  the  spindle  and  in  the  unstained  preparation  the  whole  layer  seems  to 
be  made  up  of  narrow  triangular  flecks  of  pigment. 

Finally,  upon  the  transverse  section  (PI.  VIII,  9)  the  cells  appear 
small  and  almost  quadrilateral,  the  nuclei  likewise  are  small  and  rounded; 
the  lateral  borders  of  the  cell  are  quite  as  plain  as  the  posterior  border  of 
the  whole  layer.  A  nucleus  is  not  visible  in  every  cell  in  very  thin  sections 
of  this  kind,  for  many  spindles  are  encountered  in  the  tapering  portion,  and 
the  cross-sections  of  these  are  smaller  and  lower  than  the  portions  which 
contain  nuclei.  Despite  these  and  other  smaller  irregularities  the  trans- 
verse section  shows  that  the  layer  of  pigmented  spindle  cells  is  a  single 
cell-layer;  in  other  sections  one  cannot  recognize  this  with  such  certainty. 

That  the  posterior  border  lamella  and  the  pigmented  spindle  cells 
belong  together  is  indicated  in  various  ways:  (i)  The  two  layers  are  not 
clearly  set  off  from  one  another.  (2)  The  direction  of  the  fibers  of  the 
posterior  border  lamella  and  of  those  of  the  spindle  cells  is  exactly  the 
same.  (3)  It  is  not  possible  to  separate  the  two  layers  from  each  other. 
(4)  In  many  cases  these  two  layers  cannot  be  kept  apart  in  the  section. 
According  to  Grunert  (79),  this  is  the  case  when  the  eye  has  been  under 
the  effect  of  eserin — therefore,  when  the  dilatator  is  relaxed.  Then  there 
is  only  one  layer  of  spindle  cells  visible  between  the  iris  stroma  and  the 
pigment  epithelium  but  no  border  lamella.  Grunert,  therefore,  holds 
the  posterior  border  layer  to  be  a  contraction-appearance  of  the  dilatator. 
However,  one  sees  this  reputed  contraction-appearance,  i.e.,  the  posterior 
border  lamella,  in  a  perfectly  typical  way  in  most  eyes  when,  moreover, 
these  have  not  been  under  the  eft'ect  of  atropin,  and  despite  this  the  pupil 
(as  usual)  is  pretty  small.  According  to  de  Lieto  Vollaro  (142),  the 
dilatator  elements  are  not  separated  in  the  fresh  state,  but  fused  together 
into  a  "myoid  plate." 

In  this  manner  the  dilatator  pupiUac  extends  along  the  posterior 
surface  of  the  vessel  layer  of  the  iris  from  the  ciliary  border  of  the  sphincter 
pupUlae  almost  to  the  root  of  the  iris  in  an  absolutely  uniform  develop- 
ment and  with  a  strict  maintenance  of  meridional  fibrillation.  The 
course  and  development  of  its  elements  change  only  in  the  region  of  the 
sphincter  zone  and  in  the  neighborhood  of  the  root  of  the  iris  itself. 

On  the  pupil  side  the  dilatator  has  no  sharp  border;  its  elements  finally 
go  over  into  epithelial  cells  by  means  of  incompletely  formed  fibers. 
These  transition  forms  are  partly  developed  fibers  (on  one  side,  i.e., 
cells  which  carry  a  process  on  only  one  side),  therefore,  of  a  clubbed  form 


■  This  description  is  ratlier  confined  to  the  middle  portions  of  the  iris. 


THE  IRIS  141 

(Herzog),  or  cells  with  a  wholly  irregular  form  and  pigmented  throughout 
(PI.  VIII,  10,  Di).  Some  0.2  to  0.3  mm  in  front  of  the  pupil-border 
the  outer  leaf  of  the  optic  vesicle  again  takes  on  a  purely  epithelial  char- 
acter; a  double-layered  pigment  epithelium  is,  therefore,  present  in  this 
border  zone.  But  the  anterior  layer  of  epithelium  is  not  regular  here,  by 
any  means;  some  cells  are  larger,  and  I  have  found  very  much  enlarged 
multinucleated  cells  in  this  zone,  structures  which  very  much  recall  to 
one  giant  cells.  Occasionally  attempts  at  the  formation  of  epithelial 
muscle  cells  are  also  found  here. 

Throughout  the  entire  sphincter  zone  and  its  immediate  neighborhood 
the  elements  of  the  dilatator  are  united  to  those  of  the  sphincter.  The 
former  thereby  loses  its  surface  expansion :  a  bundle  of  pigmented  fibers 
is  separated  away  from  place  to  place,  goes  over  into  a  bundle  of  ordinary 
muscle-fibers,  and  in  this  way  into  the  muscle  framework  of  the  sphincter. 
Such  radiations  are  found  at  the  ciliary  border  of  the  sphincter,  and  on 
its  posterior  surface. 

The  former  lie  at  the  pupillary  ends  of  the  structural  furrows.  On 
account  of  the  fact  that  the  structural  furrow  reaches  pretty  deep  into 
the  vessel  layer  the  dilatator  only  needs  to  continue  on  from  the  end  of  the 
furrow  in  its  original  direction  to  get  to  the  ciliary  border  of  the  sphincter. 
Such  a  bundle  consists,  peripherally,  of  pigmented  dilatator  cells,  centrally 
(in  the  neighborhood  of  the  sphincter)  of  ordinary  smooth  muscle-fibers, 
and,  just  as  the  peripheral  portion  seems  to  be  a  branching  of  the  dilatator, 
so  the  central  portion  appears  to  be  a  derivative  of  the  sphincter.  On 
surface-section  these  bundles  (PI.  VII,  6,  Sp)  seem  to  branch  off  from 
the  sphincter  like  the  spokes  of  a  wheel  from  the  axis,  and  for  this  reason 
this  bundle  is  called  the  spoke  bundle.  The  name  Michel's  pigment 
spur  has  been  derived  from  its  appearance  in  meridional  section  when  the 
pigmented  dilatator  elements  are  more  prominent  (PL  VIII,  3,  Sp). 

As  a  rule,  the  above-described  spoke  bundle  contains  the  only  radially 
directed  smooth  muscle-fibers  in  the  iris.  In  many  eyes,  however,  such 
radial  smooth  muscle-fibers  appear  farther  peripheric  and  then  form  bands 
which  strengthen  the  dilatator  piipillae;  these  are  placed  on  its  anterior 
surface,  i.e.,  that  turned  toward  the  iris  stroma.  This  variation  has  been 
more  accurately  studied  by  Widmark  (237)  and  Forsmark  (59),  who  would 
discredit  some  of  the  older  descriptions  of  the  dilatator  (or  of  the  spoke 
bundle).  Forsmark  found  such  strengthening  bands  in  a  sixth  of  his 
material. 

The  unions  of  the  dilatator  pupillae  to  the  posterior  surface  of  the 
sphincter  are  notably  weaker,  but  more  numerous  than  the  spoke 
bundles  and  consist  usually  of  a  single  or  a  few  fibers.     On  meridional 


142  ANATOMY  AND  HISTOLOGY  OF  THE  HUMAN  EYEBALL 

section  one  sees  dilatator  libers  from  place  to  place;  they  rise  up  from  the 
back  surface  and  course  through  the  firm  connective  tissue  to  the  sphincter 
in  bows  with  the  concavity  directed  forward  (PI.  VIII,  3,  e).  Yet  one 
can  seldom  follow  them  this  far  because  they  go  out  of  a  meridional 
direction.  On  transverse  section  one  obtains  a  variety  of  pictures: 
incompletely  developed  dilatator  fibers,  which  go  off  in  front,  and  sphincter 
bundles  coursing  obliquely  backward,  also  called  Fuchs's  pigment  spur 
on  account  of  their  incomplete  pigmentation,  are  seen  in  the  neighbor- 
hood of  the  border  of  the  pupil  (PI.  VIII,  10).  Farther  away  from  the 
pupillary  border  one  sees  cross-sections  of  the  dilatator  elements  in  the 
connective  tissue  often  lying  in  a  circle  about  the  vessels,  after  the  manner 
in  which  Fuchs  (67)  has  drawn  them. 

The  dilatator,  therefore,  appears  to  go  over  into  the  sphincter  every- 
where or  the  two  muscles  together  form  a  closed  framework,  probably 
without  free  endings. 

The  ciliary  border  of  the  dilatator  pupillac  about  corresponds  to  the 
place  where  the  posterior  surface  of  the  iris  bends  about  onto  the  inner 
surface  of  the  ciliary  body  (at  the  posterior  chamber  angle),  but  only 
approximately  so,  for  the  position  of  this  border  changes,  indeed,  from 
section  to  section.  At  its  ciliary  border  the  dilatator  no  longer  forms 
one  simple  lamella,  but  there  occurs  a  change  in  direction  of  the  elements: 
they  become  oblique  and  circular  in  part,  and  lie  over  one  another.  Only 
a  small  portion  maintains  the  meridional  direction  and  from  place  to  place 
they  ray  out  in  the  form  of  small  bundles  into  the  tissue  of  the  iris  root 
and  toward  the  anterior  surface  of  the  ciliary  body;  according  to  Ewing 
(58),  these  radiations  correspond  to  the  ciliary  valleys.  The  rest  of  the 
fibers  form  arcades  and  in  this  way  close  off  the  dilatator  lamella  at  the 
periphery  (PI.  VIII,  6).  Whether  a  union  with  the  ciliary  muscle  exists 
here,  as  stated  by  some  authors,  is  doubtful. 

For  the  demonstration  of  the  cihary  border  of  the  dilatator  one  must  not  tear 
the  iris  from  the  ciliary  body;  one  must  rather  so  remove  the  ciliary  body  that  the 
immediate  environment  of  the  posterior  chamber  angle  remains  still  in  connection 
with  the  iris,  and  carefully  brush  off  the  pigment  epithelium.  If  one  will  then  lay  the 
preparation  so  obtained  back  surface  up,  the  dilatator  cells  and  their  course  can  be 
recognized  by  the  peculiar  hatched  appearance  of  the  pigmentation.  The  ciliary 
border  of  the  dilatator  is  often  only  incompletely  retained  on  the  torn  iris.  Surface- 
sections  give  a  no  more  satisfactory  general  view  of  the  dilatator,  although  indi\'idual 
details,  e.g.,  the  circular  course  of  the  margin,  are  visible  in  such  sections. 

On  meridional  section  (PI.  I)  the  ciliary  border  of  the  dilatator 
presents  itself  as  a  thickening,  varying,  however,  with  every  section.  The 
arcades  come  out  as  cross-sections  of  the  bundles  of  the  spoke  cells  and 


THE  IRIS  143 

lie  for  the  most  part  in  front  of  the  meridional  coursing  fibers  when 
the  latter  are  present.  The  arcades  are  often  widely  removed  from  the 
posterior  surface  and  completely  imbedded  in  the  iris  tissue  or  in  the 
tissue  of  the  iris  root.  Its  position,  also,  is  subject  to  much  variation. 
As  soon  as  the  exclusively  meridional  direction  of  the  dilatator  element 
ceases,  the  separation  into  the  posterior  border  lamella  and  pigmented 
spindle  cells  ceases.  The  fine  unpigmented  processes  fail  in  the  cells 
of  the  border  of  the  dilatator;  its  spindle-form  body  is  more  or  less 
pigmented  throughout  its  entire  extent. 

It  is  fundamentally  incorrect  to  think  of  the  appearance  of  the  dilatator  as  always 
as  tj-pical  as  depicted.  In  the  first  place  the  structural  furrows  disturb  things,  and 
one  should,  therefore,  make  it  a  rule  to  use  sections  for  study  in  which  the  posterior 
ayers  of  the  iris  are  cut  strictly  perpendicular  to  the  surface  expanse.  Moreover, 
aside  from  such  hindrances  to  its  recognition,  it  does  not  always  show  the  same  appear- 
ance; the  influence  of  the  state  of  contraction  (Grunert)  was  emphasized  above. 
Finally,  it  lies  in  the  nature  of  the  affair,  in  the  incompleteness  of  the  differentiation 
process  which  the  dilatator  elements  have  gone  through,  so  to  speak,  that  one  meets 
different  varieties.  Remnants  of  undifferentiated  epithelium  such  as  described  by 
Grunert  and  Szili,  and  completely  developed  smooth  muscle-fibers  represent  the 
extremes  of  the  variations. 

It  is  only  in  this  way  that  one  can  understand  why  the  descriptions  of  the  various 
authors  often  vary  so  very  much  from  one  another;  only  in  this  way  is  it  conceivable 
that  strife  lasted  so  long  as  to  whether  there  was  a  dilatator  at  all,  although  its  exist- 
ence was  unconditionally  demanded  by  physiology  and  pharmacology.  This  is  not  the 
place  to  go  into  the  abundant  literature  of  the  dilatator;  the  articles  cited  in  the  text 
contain  further  references  and  are  adequate  to  orient  the  reader  concerning  the  varia- 
tion of  the  views.  The  definition  of  the  term  muscle-fiber  had  a  special  influence  upon 
this  mooted  question.  So  long  as  only  striped  and  smooth  muscles  were  known  oppo- 
nents of  the  dilatator  had  to  be,  for  as  a  matter  of  fact  a  dilatator  made  up  exclusively 
of  such  elements  does  not  e.xist.  It  was  only  when  the  knowledge  came  that  muscle- 
fibers  can  develop  out  of  epithelial  cells  that  unanimity  concerning  the  dilatator  was 
possible.  Yet  even  today  there  are  differences  in  the  conceptions  which  have  not 
been  worked  out;  for  instance,  Muench  (161)  endeavors  to  find  the  dilatator  mainly 
in  the  stroma-cell  net  of  the  vessel  layer. 

As  already  repeatedly  stated,  an  intergrowth  of  the  mesoderm  and  the  ectodermal 
elements  takes  place  in  the  development  of  the  iris.  Forsmark  has  set  up  the  view  that 
the  degree  of  differentiation  of  these  ectodermal  elements  into  muscle-fibers  depends 
upon  the  position  into  which  it  has  been  thrown  by  development;  elements  entirely 
separated  from  the  native  soil  and  wholly  imbedded  in  the  iris  stroma  grow  out  fully 
into  smooth  muscle-fibers  (the  sphincter  and  the  strengthening  band  of  the  dilatator) ; 
such  as  remain  in  situ,  on  the  other  hand,  i.e.,  on  the  surface  of  the  mesoderm,  only 
undergo  this  differentiation  in  the  basal  portion  (tj'pical  dilatator  cells).  But,  original 
as  this  conception  is,  it  can  have  no  general  value,  for  the  clump  cells  are  ectodermal 
elements  which  have  been  completely  separated  from  the  native  soil  and  have 
maintained  their  epithelial  structure  throughout. 


144  ANATOMY  AND  HISTOLOGY  OF  THE  HUMAN  EYEBALL 

5.     INNER   LEAF:    PIGMENT   EPITHELIUM   OF   THE   IRIS 

This  layer  has  an  epithelial  character  throughout.  Its  cells  are  so 
densely  filled  with  dark-brown,  round,  gross  pigment  granules  that 
neither  cell  borders  nor  nuclei  are  visible.  It  is  unconditionally  neces- 
sary that  one  study  bleached  preparations  in  order  to  come  into  a  clear 
visualization  of  the  composition  of  this  layer.  One  then  very  easily 
recognizes  the  individual  cells  as  elements  having  a  height  of  36  to  55  mu, 
a  breadth  of  16  to  25  mu;  they  are  prismatic  or  more  pyramidal  elements 
with  round  nuclei  no  larger  than  in  other  epithelial  cells  (7  mu),  and  only 
seem  strikingly  small  in  comparison  with  the  size  of  the  cells;  they  contain 
one  or  two  nucleoli  lying  close  to  the  nuclear  membrane. 

The  pigment  epithelium  seems  most  regular  in  a  transverse  section 
through  the  ciliary  zone  (PL  VIII,  9,  P) — a  simple  layer  of  uniformly 
high  cells.  Meridional  sections  through  this  zone  show  a  slight  irregular- 
ity, because  they  run  at  right  angles  to  the  circular  furrow  system  (PI. 
VIII,  8,  P).  The  furrows  appear  as  sharp  incisions,  the  rolls  between 
them  as  rounded  caps ;  in  the  neighborhood  of  the  root  of  the  iris  the  rolls 
are  higher  and  often  project  forward  a  great  deal.  The  cells  on  the 
floor  of  the  furrows  are  low  (35  mu)  and  often  wedge-form  (triangular); 
in  the  flatter  rolls  the  cells  are  simply  higher  and  somewhat  pyramidal, 
i.e.,  the  lateral  ceU  borders  diverge  toward  the  inner  surface.  Indeed,  the 
higher  the  rolls  become,  the  more  marked  duplication  of  the  epithelium 
there  is ;  the  highest  rolls  are  outspoken  folds  of  epithelium.  The  dilata- 
tor, however,  courses  smoothly  over  all  these  folds  and  furrows;  the 
circular  furrows  of  the  ciliary  zone,  therefore,  lie  only  in  the  pigment 
epithelium. 

The  same  is  true  for  parts  of  the  radial  furrows  in  the  sphincter  zone, 
namely,  for  those  in  the  immediate  neighborhood  of  the  pupil-border 
(PI.  VIII,  10,  P).  Farther  away  from  the  pupil-border  the  radial  fur- 
rows also  go  down  into  the  vessel  layer  of  the  iris  and  the  structural 
furrows  of  the  ciliary  zone  do  the  same.  In  all  these  furrows  the  pigment 
epithelium  and  the  dilatator  sink  down  in  to  an  ecjual  extent. 

The  pupil-border  of  the  pigment  epithelium  is  the  limit  of  the  optic 
vesicle,  the  place  where  the  outer  leaf  of  the  vesicle  turns  about  into  the 
inner  one,  or  the  transition  point.  As  a  rule,  this  projects  a  little  over 
the  pupillary  limit  of  the  pars  uvealis  and  bends  about  this  border  a 
bit  toward  the  front  (physiologic  ectropium  of  the  pigment  epithelium; 
PL  VIII,  3,  Ps).  For  this  reason  the  transition  area  is  visible  from  in 
front  as  a  pigment  seam  (PL  VIII,  i,  Ps).  Since  the  outer  leaf  has  a 
purely  epithelial  structure  in  the  neighborhood  of  the  transition,  the  iris 


THE  IRIS  145 

possesses  a  doubled  pigment  epithelium  in  the  neighborhood  of  the  pupil- 
border. 

In  bleached  sections  the  place  of  transition  itself  has  the  appearance 
of  the  apex  of  an  epithelial  fold  and  the  cells  in  cjuestion  are  of  wedge  shape. 
As  already  noted,  it  is  almost  in  contact  with  the  pupil-border  of  the 
sphincter,  a  permanent  indication  that  the  sphincter  pupillae  develops 
out  of  the  border  of  the  optic  vesicle. 

Many  authors  add  a  membrana  limitans  interna  to  the  description  of 
the  pigment  epithelium  of  the  iris,  similar  to  that  which  the  ciliary  epi- 
thelium possesses.  I  have,  indeed,  seen  an  extremely  thin  colorless  mem- 
brane on  the  free  surface  of  the  pigment  epithelium  in  many  preparations, 
but  it  is  unusually  delicate  and  cannot  be  isolated  over  any  considerable 
expanse. 

c)   Variations  in  the  Appearance  of  the  Iris 

{Influence  of  the  width  of  the  pupil  and  individual  variations) 

In  an  organ  possessing  such  great  mobility  as  the  iris,  the  appearance 
must  vary  according  to  the  state  of  contraction  of  its  muscle. 

The  pupil  width  may  vary  between  i .  3  and  9  mm  during  life ;  the 
average  width  is  4  mm.  These  figures  are,  however,  not  taken  from  the 
actual  pupil,  but  from  the  enlarged  image  which  the  cornea  gives  of  it. 
This  image  is  one-eighth  larger  than  the  actual  pupil  in  a  normally  deep 
chamber  of  3^  mm  and  the  actual  pupil  lies  about  o .  54  mm  farther  for- 
ward. The  above-given  limits,  therefore,  correspond  to  an  actual  pupil 
width  of  I .  I   to  8  mm. 

According  to  Albrand  and  Schroeder  (7),  the  pupil  becomes  very 
much  widened  (8  mm)  just  before  death;  then,  however,  slowly  narrows 
from  day  to  day.  Therefore,  an  average  pupil  width  is  usually  found  in 
the  cadaver.  Fixation  fluids  bring  about  a  slight  narrowing  of  the  pupil 
as  a  rule;  therefore,  it  comes  about  that  in  the  hardened  cadaver-eye  the 
pupil  width  is  usually  only  2  to  3  mm. 

It  is  not  possible  to  fix  the  pupil  at  the  extreme  widths  caused  by 
atropin  and  eserin  during  life  for  anatomic  study.  The  effect  of  these 
poisons  upon  the  pupil  passes  off  after  death  or  after  the  enucleation  of 
the  eyeball,  and  the  limits  between  which  the  pupil  can  vary  in  anatomic 
preparations  are  2  and  5 . 5  mm.  The  width  of  the  iris  (the  radius  of  the 
iris,  i.e.,  the  distance  of  the  ciliary  border  from  the  pupillary  border) 
varies  between  5  and  3  mm  (PI.  VIII,  12,  13). 

From  this  general  survey  it  is  evident  to  what  extent  we  can  accurately 
study  the  influence  of  the  width  of  the  pupil  upon  the  appearance  of  the 


146  ANATOMY  AND  HISTOLOCJ^'  OF    I'llK  HUMAN  EYKBALL 

iris;  we  can  study  the  changes  of  tlie  anterior  iris  surface  throughout  the 
full  extent  of  the  movement  of  the  i)upil,  since  these  observations  can  be 
made  in  the  living;  in  respect  to  the  posterior  iris  surface  and  the  inner 
make-up  we  are  dependent  upon  anatomic  preparations. 

Yet  the  posterior  iris  surface  can  be  studied  in  the  living  eye  by  the 
method  of  Hess  (loo);  for  the  most  part,  however,  we  are  thrown  back 
upon  anatomic  preparations  for  this,  and  with  respect  to  its  inner  make-up, 
therefore,  must  be  satisfied  with  a  lesser  play  in  the  width  of  the  pupil. 

According  to  Fuchs  (67),  the  following  changes  in  the  anterior  surface 
come  into  play  in  the  narrowing  of  the  pupil:  the  pigment  seam  of  the 
border  of  the  pupil  becomes  broader  and  its  crenation  more  plain;  the 
pupillary  zone  is  broader,  its  ridges  take  on  a  meridional  course,  the  crj'pts 
stretch  out  into  meridional  spaces,  the  angular  line  becomes  more  angular. 
When  the  sphincter  is  visible,  its  width  remains  the  same  or  increases 
somewhat;  yet  the  broadening  of  the  sphincter  is  relatively  less  than 
that  of  the  pupil  zone,  and  the  angular  line,  therefore,  moves  away  from 
the  ciliary  border  of  the  sphincter.  The  vessels  in  the  ciliary  zone  stretch 
out,  the  contraction  furrows  are  smoothed  out  and  the  border  zone  comes 
out  partially  from  behind  the  corneoscleral  border. 

On  the  posterior  surface  of  the  iris  one  notes  an  appreciable  increase 
of  the  radial  folding  at  the  pupil-border  (Hess,  139),  while  the  circular 
furrow  system  is  almost  obliterated  as  far  as  to  the  most  peripheral  furrows 
in  the  neighborhood  of  the  ciliary  border. 

On  cross-section  (PI.  VIII,  13)  the  sphincter  is  shown  to  be  broader 
and  probably  also  thicker,  and  lies  more  nearly  parallel  to  the  posterior 
surface.  The  whole  pupillary  border  is  more  bowed  about  toward  the 
front;  the  border  of  the  pigment  epithelium  covers  the  pupil-border  of 
the  sphincter  (ectropium  of  the  pigment  epithelium  becomes  more  marked). 

In  the  widening  of  the  pupil  the  pigment  seam  of  the  pupillary  border 
becomes  narrower  or  disappears  entirely,  the  pupil  zone  narrows  a  great 
deal,  and  the  anterior  or  surface  of  the  iris  falls  abruptly  away  from  the 
now  more  prominent  angular  line  toward  the  pupil-border.  The  angular 
line  is  stretched  out  and  has  almost  entirely  lost  its  angular  form;  like- 
wise the  crypts  are  drawn  out  to  oblique  clefts.  The  sphincter  has  become 
narrower  but  not  to  the  same  e.xtent  as  the  pupillary  zone,  for  the  angular 
line  has  mounted  up  over  the  sphincter.  The  vessels  in  the  ciliary  zone 
are  more  tortuous,  the  contraction  furrows  are  deeply  incised,  the  border 
zone  has  become  invisible.  The  radial  foldings  on  the  posterior  surface 
are  less  marked,  the  circular  ones  more  marked. 

On  cross-section  the  iris  seems  to  be  blunt  (PI.  VIII,  12),  the 
sphincter  appears  shorter,   narrower,   and   more  obliquely  placed,   i.e., 


THE  IRIS  147 

its  ciliary  border  is  farther  removed  from  the  posterior  surface  than  is 
the  pupillary  border.  The  pigment  epithelium  has  retracted,  and  does  not 
any  longer  cover  the  pupil-border  of  the  sphincter  (the  ectropium  has 
disappeared) . 

The  dilatator  lamella,  however,  remains  straight  in  all  widths  of  the 
pupil,  it  never  shows  foldings  or  bowings  except  where  the  iris  as  a  whole 
is  bowed  or  folded,  and  on  this  ground  alone  it  is  evident  that  the  pupil- 
dilating  principle  must  lie  in  this  layer.  The  mesodermal  layers  of  the 
iris  remain  behind,  decidedly,  in  the  widening  of  the 
pupil,  and,  therefore,  apparently  only  passively  follow 
the  pull  of  the  dilatator  lamella. 

Although  the  iris  has  doubled  its  width  in  the 
narrowing  of  the  pupil,  its  thickness  only  increases 
one-third  at  the  most,  according  to  Fuchs.  This 
striking  lack  of  relationship  seems  an  anomaly  upon  ^^^  r^Iil^s  surface 
first  thought,  but  is  very  easily  explained  by  the  fact  and  pupil  widths;  mag- 
that  the  thickness  of  the  iris  does  not  depend  upon  the     "'fi<:^t'<"i  -    Expiana- 

...  tion  in  the  text. 

breadth  ot  the  ins  but  only  upon  the  extent  of  the 

surface  expanse.     This,  however,  does  not  increase  in  the  same  ratio  as 

the  breadth  of  the  iris. 

In  Fig.  4  the  outer  circle  represents  the  fixed  periphery  of  the  iris 
(the  ciliary  border) ;  its  radius  may  be  looked  upon  as  6  mm.  The 
middle  circle  has  a  radius  of  3  mm  and  would  signify  the  maximal  width 
of  the  pupil  in  anatomic  preparation.  In  this  pupil  width  of  6  mm  the 
surface  of  the  iris  is 

6'7r-3V=(36-9)7r=27Jr. 

When  the  pupil  is  narrowed  to  2  mm  (inner  circle)  the  surface  of  the 
iris  amounts  to 

6^77-  i^7r=  (36-  l)7r  =  357r. 

In  this  the  breadth  of  the  iris  has  increased  from  3  to  5  mm,  i.  e., 
two-thirds;  the  surface  of  the  iris,  however,  not  quite  one-third.  The 
average  thickness  of  the  iris  must,  therefore,  increase  to  this  extent,  and 
this  corresponds  exactly  to  the  observations. 

Of  course  the  iris  of  the  normal  eye  is  not  completely  broadened  out 
into  a  plane,  but  this  has  no  essential  influence  on  the  relation  of  the  iris 
surfaces  in  a  narrow  and  wider  pupil. 


Finally,  in  no  other  part  of  the  eye  is  individual  variability  so  great 
as  in  the  iris.  It  is  widely  known  how  much  the  color  of  the  iris  varies, 
the  same  holds  true  of  the  thickness  of  the  iris,  of  the  relief,  of  the  internal 
structure.     A  thorough  description  of  these  varieties  is  impossible,   it 


I4S  ANATOMY  AND  HISTOLOGY  OF  THl':  HUMAN  EYEBALL 

would   eventuate   in   an   individual   description.     Moreover,    the    most 
important  variations  have  already  been  reported  in  the  proper  place. 

As  a  curiosity  only,  it  should  be  reported  that  the  variation  in  color  and  marking 
of  the  iris  called  forth  remark  even  in  antiquity,  and  finally  has  been  worked  out  to  a 
system  of  "eye  diagnosis,"  the  rankest  nonsense  which  uncritical  and  unscientific 
nature-study  in  connection  with  mysticism  and  speculation  could  bring  forth  in  the 
hands  of  credulous  laymen  and  unscrupulous  impostors  (206). 


CHAPTER  XL  THE  VITREOUS  (CORPUS  VITREUM) 

The  vitreous  as  a  whole  has  the  form  of  a  sphere  flattened  sagittally 
and  marked  in  front  by  a  round  della.  The  posterior  half  and  the  lateral 
part  of  the  surfaces  show  the  mould  of  the  shell  of  the  retina;  the  della, 
patellar  fossa  (fossa  patellaris) ,  is  the  negative  of  the  posterior  surface  of 
the  lens,  to  which  it  is  opposed.  The  transition  from  the  fossa  patel- 
laris over  into  the  outer  conve.x  surface  of  the  vitreous  presents  alow 
wall,  on  the  back  of  which  the  posterior  half  of  the  corona  ciliaris  lies  in 
undisturbed  topographic  relations  and  receives  the  shallow  radial  impres- 
sions of  the  ciliary  processes. 

The  vitreous  is  fixed  to  the  papilla  and,  less  firmly,  to  the  inner  surface 
of  the  retina,  most  firmly,  however,  at  the  ora  scrraia  and  to  the  ciliary 
epithelium  in  a  zone  some  1.5  mm  broad  immediately  adjacent  to  the 
ora  serrata.  When  the  vitreous  draws  together  under  the  influence  of 
fixation  and  hardening  fluids,  when  under  pathologic  relations  it  is 
shrunken  down  to  a  minimum — it  clings  at  this  point.  Even  severe 
injuries  do  not  tear  away  the  living  vitreous  in  this  situation,  and  when 
it  is  torn  from  it  part  of  the  ciliary  epithelium  goes  with  it,  and  the  end  of 
the  retina  loses  its  attachment. 

Since,  furthermore,  this  zone  is  of  significance  in  respects  other  than 
purely  anatomic,  it  may  be  justifiable  to  give  it  a  special  name,  and  I 
would  call  it  the  base  of  the  vitreous  (PI.  I,  Gb).  (Wolf rum,  242,  calls  this 
the  zone  of  origin  of  the  vitreous.) 

In  front  of  the  base  the  vitreous  borders  upon  a  free  space  (the  pos- 
terior chamber) ;  in  this  region  its  fixation  is  less  firm,  for  it  is  united  to 
the  bulb  wall  (the  ciliary  body)  directly  only  here  and  there  by  delicate 
processes  and  mediately  by  zonula  fibers. 

Where  the  vitreous  adjoins  the  lens,  a  more  firm  union  is  again  present 
in  the  form  of  a  ring  of  8  or  9  mm  diameter  concentric  with  the  lens  border; 
here  lies  the  ligamentum  hyaloideo-capsulare  (PI.  I,  Lhc)  described  by 
Wieger  (238). 


THE  VITREOUS  149 

The  portion  of  the  anterior  surface  of  the  vitreous  lying  inside  this 
ring  is  not  grown  to  the  lens  but  can  easily  be  drawn  away  as  soon  as  the 
resistance  of  the  ligamenhini  hyaloideo-capsulare  has  been  overcome.  A 
capillary  space  lies  between  it  and  the  lens,  the  post-lenticular  space  of 
Berger  (21). 

Nasal,  close  to  the  axis  of  the  vitreous,  courses  a  canal  (central  canal 
of  Stilling,  212;  canalis  hyaloideus  or  Cloquet's  canal).  This  begins  in 
front  of  the  papilla  in  a  funnel-form  widening  {area  martegiani)  and  courses 
through  the  vitreous  in  an  almost  sagittal  direction.  Its  width  is  i  to 
2  mm ;  the  free  end  sometimes  narrows  to  a  point  and  usually  strikes  the 
lens  nasal  to  its  posterior  pole.  According  to  Bribach  (27),  the  canal  only 
extends  4  mm  from  the  papilla  into  the  vitreous  in  some  eyes,  and  splits 
up  into  a  few  fine  branches. 

In  order  to  make  the  canal  visible  one  must  inject  it;  according  to 
Stilling,  this  often  succeeds  merely  by  simply  dropping  a  colored  solution 
onto  the  posterior  surface  of  the  freed  vitreous.  According  to  Schwalbe, 
one  may  also  obtain  a  filling  of  the  central  canal  by  an  injection  beneath 
the  pial  sheath  of  the  optic  nerve.  According  to  the  latest  demonstra- 
tion of  Stilling  (Zcitschrift  fur  Augenhcilkiinde,  Bd.  25,  S.  15),  a  narrow 
canal  is  found  in  the  mammalian  eye  only  in  the  newborn;  in  the  devel- 
oped human  eye,  on  the  other  hand,  a  wider  lymph  space  is  present; 
this  is  filled  out  by  a  soft,  tortuously  folded  substance. 

On  the  other  hand,  Wolfrum  (239)  denies  the  existence  of  a  preformed  canal  in 
this  situation,  and  explains  the  central  canal  as  an  artificial  product  arising  by  the 
tearing  off  of  the  vitreous  from  the  papilla.  In  this  matter  the  hypothesis  of  Stilling 
has  been  completely  corroborated  by  the  newer  investigations  of  Schaaff  (189)  and 
Bribach;  the  latter  has,  moreover,  published  stereoscopic  photographs  of  the  injected 
canal. 

In  the  fresh  state  the  vitreous  presents  a  completely  transparent 
entirely  colorless  mass  of  jelly-like  but  firm  consistency. 

The  vitreous  exceeds  all  of  the  other  refracting  media  of  the  eye 
in  transparency  with  the  exception  of  the  aqueous.  In  its  optical  rela- 
tions it  agrees  with  the  latter  and  with  water;  according  to  Freytag  (61), 
its  index  of  refraction  is  i .  3334  to  i  •  3350  (a  mean  of  i .  334).  Moreover, 
the  vitreous  maintains  this  transparency  a  long  time  after  death  (even 
in  a  weak  formalin  solution) ;  it  merely  becomes  thready  and  of  lessened 
consistence. 

Even  in  this  condition  the  microscopic  examination  does  not  bring 
out  the  least  trace  of  a  structure  in  the  vitreous,  and  so  it  is  conceivable 
that  in  earlier  periods,  when  none  of  the  modern  histologic  aids  were 
available,  one  did  not  understand  how  to  begin  to  study  the  vitreous. 


ISO  ANATOMY  AND  HISTOLOGY  OF  THE  HUMAN  EYEBALL 

and  that  the  most  varied  views  concerning  its  structure  were  ex[)ressed. 
Indeed,  even  the  existence  of  such  a  structure  was  denied. 

That  it  must,  however,  possess  such  is  evident  from  the  fact  that  it 
has  a  form  and  a  consistency  of  its  own;  its  elements  do  not  flow  apart 
over  its  supporting  base,  it  can  be  weighed,  etc.  (H.  Virchow,  233). 
One  can  press  fluid  out  of  it  or  filter  it  off  from  the  incised  vitreous;  it 
apparently  consists,  therefore,  of  a  framework  of  firmer  substance  and  a 
fluid  filling  out  the  meshes  of  the  latter. 

But  an  idea  of  the  arrangement  of  the  elements  of  the  framework 
can  only  be  obtained  from  hardened  preparations.  On  account  of  the 
delicacy  of  the  structure  the  sections  should  not  be  too  thin  but  very 
strongly  stained.  Retzius  (180)  used  rubin  for  this,  I  myself  have  used 
the  phospho-molybdic-acid  hematoxylin  of  Mallory,  yet  many  other 
stains  give  good  results. 

It  then  develops  that  the  whole  framework  of  the  vitreous  goes  out 
from  the  base  of  the  vitreous.  Out  of  the  somewhat  complicated  fibrilla- 
tion of  this  portion  a  thicker  layer  is  first  separated  off  and  this  extends 
along  the  inner  surface  of  the  retina  backward  as  the  posterior  border  layer 
(PI.  I,  liG).  Even  by  low-power  magnification  one  sees  a  fine  striation 
parallel  to  the  surface  and  usually  slight  waviness,  which  is  probably  the 
result  of  the  shrinking  of  the  tissue  in  the  imbedding — a  peculiar  picture 
which  one  does  not  find  in  any  other  tissue  of  the  eye  and  which  I  can 
only  compare  with  the  flowing  hair  of  a  woman.  The  posterior  border 
layer  is  thickest  immediately  at  the  ora  serrata;  farther  backward  it  is 
always  thinner,  since  the  innermost  layers  constantly  turn  off  into  the 
interior  of  the  vitreous.  In  this  way  the  posterior  border  layer  finally 
breaks  up  entirely  and,  indeed,  the  border  layer  is  in  pretty  firm  union 
with  the  mcmbrana  limitans  interna  retinae  opposite  the  entrance  of  the 
optic  nerve  in  the  area  martegiani,  and  when  the  vitreous  is  detached  as 
a  result  of  the  hardening  the  limitans  follows,  as  a  rule. 

That  portion  of  the  fibrillation  of  the  base  of  the  vitreous  which  does 
not  enter  the  posterior  border  layer,  radiates  into  the  interior  of  the 
vitreous,  broadening  out  like  a  fan,  and  forms  the  loose,  often  shreddy, 
body  of  the  vitreous,  along  with  the  branches  of  the  border  layers. 

As  above  reported,  the  anterior  border  of  the  base  of  the  vitreous  lies 
about  1 . 5  mm  in  front  of  the  ora  serrata,  that  is,  the  more  compact  mass 
of  the  base  of  the  vitreous  is  inserted  as  a  wedge  about  this  distance  away. 
Yet  a  very  loose  framework  extends  still  farther  forward  to  the  middle  of 
and  even  beyond  the  orbiculus  ciliaris  and  then  is  gradually  lost. 

About  the  middle  of  the  orbiculus  ciliaris,  therefore  about  2  mm  in 
front  of  the  ora  serrata,  a  thickening  of  the  framework  of  the  vitreous 


THE  VITREOUS  151 

again  appears.  This  often  begins  with  a  pretty  sharp  border  in  the 
above-reported  looser  framework  and  stretches  out  from  there  over  the 
whole  of  the  anterior  surface  of  the  vitreous.  It  separates  the  vitreous 
from  the  posterior  chamber  and  from  the  lens  and  is,  therefore,  called  the 
anterior  border  layer  (PI.  I,  vG).  Like  the  posterior  border  layer  it 
shows  a  striation  parallel  to  the  surface  and  is  easily  thrown  into  circular 
folds. 

The  anterior  border  layer  is  much  thinner  than  the  posterior  and  main- 
tains this  thickness  unchanged  up  to  the  border  of  the  lens ;  it  then  thins 
out  in  the  territory  of  the  lens  and  in  the  middle  of  the  patellar  fossa 
reaches  a  minimum  without  wholly  dissolving  away.  As  in  the  posterior 
border  layer,  the  thinning  comes  about  by  a  variation  off  into  the  body  of 
the  innermost  layers. 

Naturally,  the  anterior  border  layer  is  sharply  bordered  off  from 
the  posterior  chamber,  but  it  is  also  much  more  sharply  set  off  inward 
(toward  the  body  of  the  vitreous)  than  is  the  posterior  border  layer.  It 
therefore  gives  the  impression  of  a  membrane  in  many  eyes  in  cross- 
section,  especially  when  it  has  been  compressed  by  hardening  and  when  its 
thickness  does  not  exceed  that  of  the  lens  capsule.  This  is  the  reason 
why  many  authors  speak  of  a  hyaloidea  in  this  section  of  the  vitreous. 
This  hyaloidea  is  identical  with  the  anterior  border  layer.  Proof  of  this 
is  furnished  by  its  histologic  structure,  and  this  will  be  discussed  later. 

The  anterior  border  layer  can  easily  be  prepared  as  a  continuous 
coat,  and  when  laid  out  flat  it  presents  a  circular  disk  about  18  mm  in 
diameter. 

To  prepare  the  anterior  border  layer  one  chooses  an  eye  which  has  been  well  fixed 
in  Mueller's  fluid,  halves  it  at  the  equator,  and  carefully  removes  the  loose  shreds  of  the 
vitreous  body  from  the  anterior  segment.  If  one  can  keep  from  touching  the  wall  of 
the  eye  in  doing  this  the  body  can  be  comi)letely  removed  without  injuring  the  border 
layer. 

One  now  separates  the  retina  bluntly  from  the  ora  serrala  and  carefully  removes 
everything  remaining  in  connection  with  the  retina.  A  greater  resistance  is  felt  first 
at  the  posterior  border  of  the  corona  ciliaris  and  then  at  the  border  of  the  lens,  yet 
this  can  be  overcome  by  pulling  on  the  already  detached  portions. 

The  whole  of  the  tissue  detached  in  this  way  is  stained  deeply,  incised  radially, 
and  laid  out  flat.  Aside  from  the  entire  anterior  border  layer,  such  a  preparation 
contains  the  posterior  half  of  the  zonula,  the  base  of  the  vitreous,  and  the  border  portion 
of  the  retina.  A  simple  pull  backward  on  the  base  of  the  \atreous  completely  separates 
the  border  layer  from  the  portion  clinging  to  it. 

Prepared  free  in  this  way,  the  anterior  border  layer  shows  many 
circular  folds;  in  the  region  of  the  corona  ciliaris  the  impressions  of  the 
ciliary  processes  are  visible  as  flat  meridional  furrows,  and  between  these 


IS2  ANATOMY  AND  HISTOLOGY  OF  THE  HUMAN  EYEBALL 

there  sometimes  lie  delicate  membranous  clinging  structures  (the  "Uga- 
mentes  cordiformes"  of  Campos,  30),  which  likewise  have  a  meridional 
course.  A  portion  of  the  zonula  fibers  have  clung  to  the  border  laj-er 
{innermosl  zonula  fibers;  see  chap,  xii),  or  have  left  fine  canals  impressed 
on  it;  individual  zonula  fibers  ray  out  into  the  border  layer  and  go  over 
into  circular  fibers.  Where  the  border  layer  clings  to  the  lens,  a  darker 
ring  sometimes  comes  out  and  the  stump  of  the  ligamenliim  hyaloidco- 
capsulare  clings  to  it  as  a  delicate  circular  fringed  seam.' 

The  most  important  difference,  however,  between  the  posterior  and 
the  anterior  border  layer  is  that  the  posterior  border  layer  is  connected 
with  the  membrana  Umitans  interna  retinae  throughout,  while  the  anterior 
border  layer  is  separated  from  the  analogous  membrana  Umitans  interna 
ciliaris  by  the  posterior  chamber.  The  separation  is  not  everywhere  a 
complete  one,  in  so  far  as  the  posterior  chamber  is  in  places  crossed  by 
delicate  extensions  of  the  vitreous. 

So  the  posterior  zone  of  the  anterior  border  layer  is  nevertheless  very 
much  connected  with  the  membrana  lifnitans  interna  ciliaris  and,  indeed, 
by  means  of  fine  vitreous  fibrillae,  which  course  from  behind  and  within, 
forward  and  outward,  and  wind  in  and  out  tortuously  between  the  zonula 
fibers  which  are  numerous,  even  here. 

Another  system  of  vitreous  extensions  appears  in  the  posterior  part 
of  the  corona  ciliaris.  Corresponding  to  each  ciliary  valley  one  often 
sees  here  a  wing-like  process  directed  meridionally,  elevating  itself  out  of 
the  anterior  border  layer  from  a  three-sided  prismatic  base;  it  then 
divides  up  into  very  tortuous  bands  and  finally  into  finest  fibrillae,  and 
so  ends  at  the  membrana  Umitans  interna  ciliaris.  These  processes  are 
easiest  seen  in  transverse  sections  going  through  the  corona  ciliaris  (PI. 
VII,  2,  Lc),  and  in  them  I  think  I  have  recognized  the  ligamentes  cordi- 
formes described  by  Campos  (30). 

A  structure  united  to  the  lens  capsule  but  not  with  the  Umitans 
interna  ciliaris  and  likewise  belonging  in  the  category  of  these  extensions 
of  the  vitreous  is  the  already  reported  ligamentum  hyaloideo-capsulare 
(Wieger,  238). 

Just  in  front  of  the  place  where  the  anterior  border  layer  reaches  the 
lens  capsule,  a  delicate  fibrillar  mass  (PL  I,  LIic)  rises  up  out  of  the 
border  layer  on  a  three-sided  prismatic  base  and,  diminishing  in  bulk, 
extends  along  through  the  zonula  fibers  to  the  lens  capsule.  It  can  be 
followed  as  far  as  the  equator  of  the  lens  or,  indeed,  even  beyond  this 
to  the  anterior  zonula  fibers.     Its  appearance  has  been  described  above 


■  With  respect  to  this  and  many  other  details  not  noted  here  I  refer  the  reader  to  my  monograph 
(184). 


THE  VITREOUS  153 

in  the  surface  preparation  of  the  border  layer;  the  dark  ring  at  times 
visible  at  this  place  does  not  really  come  from  the  ligament  itself  but 
from  a  thickening  of  the  border  layer. 

I  have  allowed  the  name  used  by  Wieger  for  this  structure  to  remain,  because  this 
author  is  the  only  one  who  describes  a  special  substratum  structure  for  the  fixation  of 
the  \'itreous  to  the  lens.  If  his  description  differs  to  any  extent  from  my  own,  this  is 
mainly  due  to  the  difference  in  technique.  The  name  does  not,  indeed,  fall  in  with  the 
nomenclature  used  by  me,  because  I  cannot  look  upon  the  border  layer  as  the  hyaloidea, 
as  Wieger  does.  It  only  surjDrises  me  that  no  one  else  has  reported  this  extension  of 
the  vitreous ;  as  a  matter  of  fact  it  is  very  easily  seen  after  ordinary  staining. 

It  does  not  need  to  be  proven  that  this  structure  has  a  relationship  to  the  fetal 
tunica  vasculosa  lentis,  for  throughout  it  corresponds  in  its  position  to  this  fetal  struc- 
ture. When,  however,  I  speak  of  this  as  a  remnant  of  the  vascularized  fetal  lens 
capsule,  it  is  not  to  be  taken  literally.  The  ligament  does  not  arise  out  of  the  vessels  of 
this  capsule;  ectodermal  vitreous  tissue  has  simply  developed  along  these  vessels. 

It  is  made  clear  from  this  description,  therefore,  that  the  vitreous 
possesses  throughout  a  thicker  rind  or  border  layer.  This  fails  in  only 
two  places:  one  is  the  area  martegiani,  the  entrance  to  the  central  canal; 
the  other  lies  between  the  base  of  the  vitreous  and  the  margin  of  the 
anterior  border  layer.  This  latter  has  the  form  of  a  circular  cleft  and  on 
account  of  its  relation  to  a  portion  of  the  zonula  fibers  I  have  called  this 
place  the  zonular  cleft  (184)  (PL  I,  Z).  The  zonular  cleft  is,  however, 
not  a  free  open  space,  but  only  a  place  where  the  delicate  framework  of 
the  body  of  the  vitreous  comes  out  onto  the  surface.  Whether  or  not, 
as  Kuschel  (153)  thinks,  it  is  an  efferent  for  the  fluid  of  the  vitreous 
must  first  be  established  by  experimental  means. 

The  question  whether  or  not  the  vitreous  has  a  surrounding  mem- 
brane of  its  own,  a  hyaloidea,  has  long  been  a  matter  of  contention.  In 
post-mortal  or  artificial  detachment  of  the  vitreous  from  the  retina,  in  an)- 
case,  one  sees  a  plain  membrane  on  its  inner  surface  as  a  rule,  but  this 
is  nothing  more  than  the  membrana  limitans  interna  retinae  (see  p.  79). 
It  is  a  matter  open  to  discussion  whether  this  covering  belongs  to  the 
retina  or  to  the  vitreous,  and  in  keeping  with  this,  one  may  call  it  either  the 
membrana  limitans  interna  retinae,  or  the  hyaloidea  as  the  case  may  be; 
but  one  thing  is  certain:  aside  from  this,  no  other  (second)  membrane 
exists  between  the  bases  of  the  Mueller's  fibers  and  the  border  layer  of  the 
vitreous.  

The  vitreous  tissue  consists  of  a  framework  of  extremely  delicate  and 
soft  fibrillae  and  of  a  fluid  lying  in  interspaces.  Both  substances  must 
possess  exactly  the  same  refracting  capacity,  otherwise  it  would  be  impos- 
sible to  account  for  the  complete  transparency  of  the  vitreous. 


154  ANATOMY  AND  HISTOLOGY  OF  THE  HUMAN  EYEBALL 

Living  fixed  cells,  blood-vessels,  and  nerve-fibers  are  entirely  absent 
in  the  fully  formed  vitreous. 

Body  (nucleus)  and  rind  (border  layer)  are  differentiated  particularly 
by  the  density  of  the  framework  and  their  texture.  The  nucleus  is  best 
studied  in  teased  preparations:  one  tears  out  the  loose  body  shreds  with  a 
pincette  and  stains  them  a  few  hours  with  Mallory's  hematoxylin.  High 
power  then  shows  numerous  fine  fibrillae  closely  interwoven  and  forming 
a  tissue  having  the  appearance  of  a  spider-web.  The  fibrillae  are  immeas- 
urably fine  and  extremely  soft,  and  the  whole  framework  can  be  very 
easily  distorted.  Angular  granules  are  found  strewn  about  here  and 
there  in  this  framework,  and  are  considered  by  Wolfrum  (239)  to  be  the 
end  processes  of  the  embryonic  radial  vitreous  fibers  (cf.  chap.  xvi). 
Moreover,  one  ordinarily  sees  a  fine  granular  precipitate  upon  the  fibrillae. 
This  probably  arises  out  of  the  fluid  of  the  vitreous. 

Since  such  a  vitreous  shred  is  drawn  out  when  it  is  torn  away  and 
bruised  by  the  pincette  and  pressed  down  fiat  by  the  coverslip,  such  a 
preparation  can  give  only  an  approximate  idea  of  the  structure  of  the 
fibrillae  and  their  number.  The  actual  texture  of  the  body  of  the  vitreous, 
the  interweaving  of  the  fibrillae  in  all  three  directions,  in  short,  the 
framework  which  it  forms,  can  only  be  made  out  in  sections;  these, 
however,  must  not  be  too  thin,  because  of  the  delicacy  of  the  tissue.  On 
the  other  hand,  the  border  layers  consist  of  a  large  number  of  very  delicate 
superficially  parallel  lamellae,  and  the  striation  which  they  show  upon 
cross-section  is  only  the  optical  cross-section  of  these  lamellar  systems 
and  appears  the  same  whether  the  section  is  meridional  or  transverse. 
The  lamellation  of  the  border  layer  comes  out  very  plainly  in  surface  and 
teased  preparations.  In  the  first  place,  there  is  no  difficulty  in  splitting 
the  border  layer  parallel  to  the  surface,  and  each  of  the  membranes 
obtained  in  this  way  furthermore  shows  a  large  number  of  lamellae, 
which  one  can  recognize  in  microscopic  study,  especially  by  the  terrace 
formation. 

Each  lamella  consists  of  a  net  of  fibrillae.     These  are  much  finer  even 
than  the  fibrillae  of  the  body.     They  are  so  fine  that  the  net  cannot  be' 
completely  traced  out  optically,  even  with  the  strongest  magnifications, 
although  the  lamellae  do  not  appear  homogeneous  even  in  moderately 
high  magnifications. 

When  the  individual  lamellae  are  followed  from  within  outward,  an 
increasing  fineness  of  the  elements  and  an  increasing  density  of  the  net  is 
made  out.  Yet  the  most  peripheric  lamella — that  bordering  immediately 
on  the  posterior  chamber — is  not  entirely  homogeneous,  although  it 
acts  very  much  like  a  homogeneous  membrane  in  all  of  its  relations. 


THE  ZONULA  CILIARIS  155 

The  angular  granules  seem  to  be  entirely  absent  in  the  border  layers; 
on  the  other  hand,  living  cells  are  found  on  the  outer  surface  of  the  pos- 
terior border  layer  and  at  the  vitreous  base  also.  These  have  been  called 
the  subhyaloid  cells  by  some  authors  (PI.  V,  8).  They  do  not  form  a 
continuous  coat.  They  are  wholly  isolated  cells  separated  by  fairly  wide 
interspaces,  provided  with  simple  or  fragmented  nuclei  and  a  granular 
protoplasm.  It  is  relatively  seldom  that  these  cells  are  round;  for  the 
most  part  they  show  several  processes,  often  possess  node-form  or  vesicular 
thickenings.  When  the  protoplasm  contains  larger  vesicles  with  a  clear 
content,  they  are  called  physaliphores.  All  these  cell  forms  are  to  be 
considered  wandering  cells;  the  absence  of  the  closed  union  and  the 
presence  of  amoeboid  movements,  which  Iwanoff  (113)  saw  in  them  in  the 
living  vitreous,  speak  for  this. 

There  are  no  membranes  inside  the  vitreous.  A  few  authors,  as  Straub  (215), 
consider  the  vitreous  to  be  made  up  of  a  large  number  of  membranes,  but  the  con- 
ception apparently  has  its  origin  in  unsuitable  fixation;  in  pathologic  cases  also  the 
fibrillae  so  press  together  that  a  membrane  is  simulated. 

According  to  Addario  (3),  the  vitreous  fibrillae  come  out  of  the  apices  of  the  inner 
extensions  of  the  ciliary  epithelium  (cf.  p.  122),  and  on  this  basis  he  considers  the  pos- 
terior zone  of  the  ciliary  epithelium  to  be  the  matrix  of  the  \dtreous.  There  is  much 
to  be  said  for  this  view:  the  firm  unions  of  these  parts,  the  development  of  the  defini- 
tive vitreous,  the  deleterious  influence  which  inflammations  of  the  ciliary  body  have 
upon  the  vitreous;  but  the  transition  of  the  vitreous  fibrillae  into  the  ciliary  epithelium 
in  the  adult  eye  probably  should  not  be  taken  so  literally;  the  two  kinds  of  tissue  are 
sharply  separated  from  one  another. 

Tornatola  (224)  even  conceives  of  a  connection  of  the  vitreous  fibrillae  with 
Mueller's  supporting  fibers  of  the  retina.  Whoever  believes  in  the  existence  of  the 
membrana  limitans  interna  retinae  cannot  agree  with  the  view  of  Tornatola  (cf.  p.  79). 


CHAPTER    XII.     THE    ZONULA    CILIARIS     (Z.    ZINNII,    LIGAMENTUM 
SUSPENSORIUM  LENTIS) 

By  the  zonula  one  understands  that  system  of  fibers  spanned  out 
between  the  inner  surface  of  the  ciliary  body  on  the'  one  side  and  the 
equatorial  zone  of  the  lens  and  the  anterior  border  layer  of  the  vitreous 
on  the  other  side ;  it  mainly  fixes  the  lens  in  its  position  and  has  an  effect 
upon  the  form  of  the  lens  through  the  function  of  the  ciliary  body. 

In  order  to  obtain  a  general  view  of  the  zonula  one  looks  at  the  anterior 
segment  of  the  eyeball  from  behind  under  intensely  focalized  light  and 
moderate  magnification  (PL  IV,  10).  One  then  perceives,  even  in  the 
anterior  portion  of  the  orhiculns  ciliaris,  a  fine,  silk-like,  shiny  meridional 
striation  (better  seen  in  hardened  eyes  than  in  fresh  ones),  which  extends 
into  the  ciliary  valleys  but  leaves  the  processes  free.    From  the  middle  of 


iS6  ANATOMY  AND  HISTOLOGY  OF   THK  HUMAN  EYEBALL 

the  corona  on,  fine,  shiny,  still  very  plainly  separated  libers  come  out 
of  the  ciliary  valleys  close  to  the  processes  and  course  over  the  anterior 
part  of  the  corona  through  the  circumlental  space  to  the  posterior  surface 
of  the  lens.  The  fibers  are  apposed  to  the  side  surfaces  of  the  processes, 
so  that  they  appear  to  come  from  them,  and  each  ciliary  process  is  flanked 
by  two  rows  of  zonula  fibers.  A  similar  picture  is  obtained  in  the  view 
from  the  front  after  the  removal  of  the  iris:  two  fiber-bundles  appear 
to  run  from  each  ciliary  process  to  the  anterior  surface  of  the  lens. 

The  number  of  the  processes  is  something  like  70,  so  the  zonula  is 
divided  up  into  some  140  meridional  rows  or  lamellae  (Retzius,  180) 
apposed  to  the  ridges  of  the  ciliary  processes  in  the  circumlental  space 
(the  free  portion  of  the  zonula). 

In  order  to  obtain  the  zonula  in  a  single  preparation,  one  proceeds 
as  in  the  preparation  of  the  anterior  border  layer.  When  one  makes  this 
preparation  from  the  hardened  eye  only  the  posterior  half  of  the  zonula 
— that  from  the  ora  serrata  up  to  the  posterior  border  of  the  corona  ciliaris — 
comes  into  view.  In  the  fresh  eye,  on  the  other  hand,  the  zonula  can  be 
prepared  as  a  whole  in  this  way,  whereby,  of  course,  the  limUans  interna 
ciliaris  and  more  or  less  of  the  epithelial  covering  of  the  ciliary  body 
comes  away  with  it. 

In  this  way  one  is  very  easily  convinced  that  the  posterior  border  of  the 
zonula,  in  so  far  as  it  forms  a  closed  fiber  layer,  lies  some  i .  5  mm  in  front 
of  the  ora  serrata,  therefore  at  the  anterior  border  of  the  base  of  the  vitreous. 
This  border,  the  posterior  zonula  border  of  many  authors,  reproduces  the 
form  of  the  ora  serrata  in  such  a  way  that  the  border  of  each  tooth  is 
correspondingly  withdrawn  from  the  corona,  and  the  finer  undulations  of 
the  border  of  the  retina  correspond  to  the  marked  zig-zag  outline  of  the 
posterior  border  of  the  zonula.  Like  the  large  mesh  zone  of  the  reticulum 
of  H.  Mueller  (p.  118),  the  posterior  border  layer  is,  therefore,  a  carica- 
ture of  the  ora  serrata.  But  this  posterior  limit  of  the  zonula  is  not  the 
limit  of  all  the  fibers.  A  minority  of  the  fibers  come  from  points  lying  still 
farther  back,  and,  indeed,  a  good  many  from  the  inner  surface  of  the  ciliary 
body  in  the  region  of  the  base  of  the  vitreous,  and  some  few  out  of  the 
vitreous  itself,  partly  as  radiations  into  the  body  of  the  vitreous,  partly 
(on  the  nasal  side)  out  of  the  posterior  border  layer  in  the  neighborhood  of 
the  ora  serrata  (PI.  I). 

All  of  these  fibers  arising  out  of  the  vitreous  or  passing  through  the 
base  of  the  vitreous  course  through  the  zonular  cleft  and  unite  with  the 
main  mass  springing  from  the  posterior  border  of  the  zonula.  So  many 
fibers  arise  out  of  this  that  a  closed  layer  of  meridionally  coursing  fibers 
covering  up  all  the  others  at  once  forms  and  stretches  over  the  whole 


THE  ZONULA  CILIARIS  157 

orbicidiis  ciliaris  in  this  manner,  lending  to  it  tlie  above-reported  fine 
striation. 

If  one  wislies  to  be  further  oriented  concerning  the  origin  of  the 
zonula  fibers  and  their  topographical  relations  one  must  turn  his  attention 
to  sections.  Such  preparations  then  show  that,  with  the  exception  of  the 
whitish  crests  of  the  processes,  fine  zonula  fibers  also  go  off  from  the  entire 
inner  surface  of  the  ciliary  body  as  far  as  the  sims  and  unite  with  those 
coming  from  behind,  already  grown  to  larger  fibers.  So  it  comes  about 
that,  in  general,  the  larger  zonula  fibers  lie  inward  over  the  orbiculus,  i.e., 
toward  the  side  of  the  vitreous,  the  finer  fibers,  on  the  other  hand,  outward, 
i.e.,  on  the  side  of  the  ciliary  epithelium  (PI.  VII,  2,  5,  Z). 

The  direction  of  these  fine  fibers  (auxiliary  fibers,  Garnier,  72)  is, 
in  general,  the  same  as  that  of  the  larger  ones — from  behind  and  without, 
forward  and  inward;  over  the  orbiculus  they  course  almost  parallel  to  its 
inner  surface;  toward  the  front  they  mount  up  more  and  more  abruptly, 
and  the  most  anterior  are  placed  nearly  perpendicular  to  the  inner  surface 
of  the  ciliary  body. 

Moreover,  aside  from  these  straight  coursing  fibers,  fibers  which  course 
backward  are  found  in  the  anterior  part  of  the  orbiculus  and  the  posterior 
part  of  the  corona;  these  were  called  the  orbiculo-ciliary  fibers  by 
Czermak  (34),  i.e.,  fibers  which  course  from  without  and  in  front  back- 
ward and  inward.  There  arises  thereby  a  narrow-angled  crossing  of  the 
fine  zonula  fibers  very  well  seen  in  meridional  section  (Taf.  I),  for  example, 
just  behind  the  corona  ciliaris.  This  double  anchorage  brings  about  a 
stronger  fixation  of  the  entire  zonula  fiber  mass  at  the  posterior  border 
of  the  corona  ciliaris,  and  this  is  the  reason  why  with  few  exceptions  the 
zonula  fibers  break  away  at  this  place. 

Those  which  do  not  are  relatively  weak  fibers  coursing  close  to  the 
anterior  border  layer  of  the  vitreous,  and  often  imbedded  in  canals  in  the 
border  layer;  for  this  reason  I  have  called  them  the  innermost  fibers 
(posterior  zonula  bundle  of  Retzius,  180;  central  layer  of  Graf  Spee, 
210).  They  come  from  far  back,  have  no  fibers  uniting  them  to  the  inner 
surface  of  the  ciliary  body,  and  have  nothing  to  do  with  the  divisions  of 
the  latter;  they  are  the  only  fibers  which  even  occasionally  course  along 
the  crests  of  the  ciliary  processes  (PI.  VII,  2,  iz);  they  end  at  the 
posterior  surface  of  the  lens  or  in  the  border  layer  of  the  vitreous. 

The  rest  of  the  zonula  fibers  vary  away  from  the  processes  and  course 
into  the  valleys.  The  whole  complex  of  zonula  fibers  entering  into  one 
ciliary  valley  thus  divides  into  two  halves,  which  then  course  farther  on 
along  the  side  surfaces  of  the  processes  and  finally  go  over  into  the  free 
part  of  the  zonula  in  meridional  rows. 


158  ANATOMY  AND  HISTOLOGY  OF  THE  HUMAN  EYEBALL 

The  outline  of  each  row  forms  a  triangle,  the  apex  of  which  lies  in  the 
posterior  part  of  the  corresi)onding  ciliary  valley  and  its  base  in  the 
equatorial  zone  of  the  lens  capsule.  The  anterior  surface  of  the  triangle 
is  straight;  the  posterior  surface  describes  a  bow  apposed  to  the  wall 
about  the  fossa  pateUaris. 

The  triangle  is,  however,  by  no  means  uniformly  filled  out  with 
zonula  fibers,  as  O.  Schultze  (197)  states,  but  the  fibers  are  massed  toward 
the  anterior  and  the  posterior  sides  and  a  much  smaller  number  of  fibers 
course  in  the  middle  portion.  The  anterior  fibers  are  the  largest  and  are 
inserted  in  front  of  the  equator  of  the  lens;  they  form  the  main  anterior 
train  of  fibers  (Topolanski,  222)  and  the  totality  of  the  main 
anterior  train  of  fibers,  in  all  about  140  rows,  forms  the  anterior 
zonular  leaf  (PI.  I,  vZ). 

The  posterior  fibers  are  finer  than  the  anterior;  they  unite  or  cross 
over  with  the  most  anterior  ones  coming  from  the  corona,  and  in  this  way 
they  form  a  double  fan,  or  a  figure  similar  to  a  row  of  tangents  to  a  curve 
or  an  evolute  (Schoen,  192).  These  fibers,  whose  insertion  is  on  the 
posterior  surface  of  the  lens,  i.e.,  lies  behind  the  equator  lentis,  form 
the  main  posterior  train  of  fibers  (Topolanski),  and  their  totality  forms  the 
posterior  zonular  leaf  (PI.  I,  hZ). 

The  middle  or  equatorial  fibers  are  inserted  into  the  equator  of  the  lens 
or  in  the  immediate  neighborhood  of  it.  They  are  the  weakest  of  all  and 
least  regularly  developed  (PI.  I,  qZ). 

This  description  depicts  the  type  or  the  principle  of  arrangement  of 
the  zonula.  It  is  fundamentally  incorrect,  however,  to  e.xpect  to  find  this 
principle  carried  out  with  mathematical  accuracy.  To  begin  with,  the 
individual  rows  are,  indeed,  strictly  meridional;  the  irregularity  of  the 
ciliary  processes  makes  the  individual  fibers  go  out  of  the  row,  and  finally 
the  fibers  diverge  after  their  exit  from  the  ciliary  bodies  in  a  frontal  direc- 
tion as  well,  or  they  branch  in  this  direction. 

When  one  looks  at  a  very  thick  section — one  comprising  several  ciliary 
valleys — under  weak  magnification,  it  does,  indeed,  seem  as  if  the  zonula 
fibers  uniformly  fill  out  the  entire  space  which  the  older  anatomists  called 
Petit's  canal  (O.  Schultze,  197).  But  when  one  carries  through  a  section 
tangential  to  the  equator,  it  is  at  once  seen  that  the  cross-sections  of 
zonula  fibers  are  not  uniformly  distributed  through  this  space,  but  are 
at  least  thicker  along  the  anterior  and  posterior  sides.  The  study  of  the 
insertion  zone  on  the  lens  leads  to  the  same  result  (cf.  chap.  xiii). 

No  description  is  able  to  give  with  absolute  accuracy  any  intermediate  concep- 
tion between  the  regularity  and  irregularity  of  arrangement  which  characterizes  the 
organized  structure.     The  one  emphasizes  the  plan  of  the  whole  arrangement  and 


THE  ZONULA  CILIARIS  159 

this  description  then  naturally  emphasizes  the  reguhirity;  the  other  sees  only  the 
variations  from  the  rule  and  for  this  reason  denies  any  law.  The  truth  lies  in  the 
mean  between  the  two  extremes. 

All  of  the  heretofore  described  elements  of  the  zonula  (and  these 
form  by  far  the  greater  majority)  possess  a  meridional  direction  or  vary 
only  insignificantly  from  this  direction.  Moreover,  the  majority  unite 
the  inner  surface  of  the  ciliary  body  to  the  equatorial  zone  of  the  lens. 

Although  then,  we  look  upon  these  as  the  most  important,  function- 
ally, and  accordingly  designate  them  as  the  typical  zonula  fibers,  there 
still  remains  a  minority  of  fibers  having  another  direction  and  united  with 
other  structures;  these  we  will  call  atypical  zonula  fibers  for  short, 
from  which,  however,  it  is  not  to  be  understood  that  they  are  to  be  con- 
sidered of  no  significance  functionally. 

To  the  atypical  zonula  fibers  belong  in  the  first  place  those  coming 
out  of  the  vitreous  and  the  backward-coursing  zonula  fibers,  which  we  have 
already  discussed.  Here,  too,  belong  the  inter-  and  intraciliary  fibers 
of  Czermak  (34),  fine  fibers  uniting  two  ciliary  processes  with  each  other. 
Furthermore,  the  short  and  thick  fibers  going  out  of  the  corona  ciliaris 
directed  at  right  angles  to  the  typical  fibers  and  serving  for  the  fixation  of 
these,  described  by  Retzius  (180),  belong  here.  But  in  my  opinion  these 
fibers  do  not  unite  with  the  t>'pical  fibers,  but  radiate  into  the  border  layer 
of  the  vitreous  or  go  over  into  circular  fibers. 

For  if  one  makes  an  anatomic  preparation  of  the  border  layer  of  the 
vitreous  in  the  manner  heretofore  described  and  looks  at  the  surface,  one 
sees  a  varying  number  of  zonula  fibers  radiate  into  it  (PI.  VIII,  14). 
One  part  of  these  fibers  belongs  to  the  innermost  zonula  fibers  (see  p. 
157);  the  other  part  {k)  consists  of  shorter  or  longer  stumps,  which 
apparently  corrie  from  the  corona  ciliaris,  for  the  radiations  into  the 
vitreous  lie  in  this  region.  The  individual  fibers  are  thereby  broadened 
out  into  the  form  of  a  narrow  triangle  and  then  break  up  into  a  large 
number  of  fine  divergent  processes.  At  times  only  a  part  of  the  fibers 
radiate  into  the  vitreous  layer,  and  the  remainder  course  on  farther  to  the 
lens,  or  one  fiber-bundle  takes  root  at  two  or  three  places  in  the  vitreous 
body  in  this  way  (w).  The  greater  part  of  the  fiber  processes  very  soon 
lose  themselves  in  the  border  layer,  the  other  part  bends  about  in  a  sharp 
curve  into  the  circular  course. 

These  circular  zontila  fibers  {cZ)  sometimes  stretch  over  a  great  part 
of  the  border  layer,  always  following  the  corona  ciliaris  or  rather  the  wall 
which  lies  around  the  fossa  patellaris.  The  border  layer  itself  is  always 
pretty  strongly  folded  in  such  places  and  for  this  reason  one  is  not  able 
to  follow  the  individual  circular  zonula  fibers  over  wide  stretches.     When, 


iCo  ANATOMY  AND  HIS  r()LOG\'  OF  THE  HUMAN  EYEBALL 

however,  two  radiations  lie  close  to  one  another,  a  few  fibers  of  union  can 
very  easily  be  made  out,  and  it  is  to  be  suspected  that  the  most  of  the 
circular  fibers  sooner  or  later  go  over  again  into  t^'pical  ones,  i.e.,  into 
meridional-coursing  zonula  fibers. 

The  transition  of  meridional  into  circular  fibers  can  only  be  plainly 
seen  in  surface  preparation  of  the  anterior  border  layers;  the  circular  fibers 
themselves  may,  however,  also  be  seen  in  meridional  sections,  for  example, 
when  they  are  numerous  and  large,  for  the  cross-section  of  the  zonula 
fibers  stand  out  clearly  on  account  of  their  size  and  characteristic  appear- 
ance in  contrast  with  the  finer  fibrillation  of  the  border  layer  itself.  They 
lie  in  part,  certainly,  in  the  border  layer  and  not  simply  upon  it.  A  strong 
folding  of  the  border  layer  indicates  the  place  where  circular  zonula  fibers 
are  to  be  found  (PI.  I,  cZ).  In  transverse  sections  of  this  region  the 
circular  fibers  appear  cut  along  their  length,  even  on  weak  magnification, 
and  stand  out  thereby  from  the  typical  fibers,  which  are  cut  across, 
throughout. 

The  circular  zonula  fibers  have  already  been  incidentally  described  by  various 
earlier  authors,  yet  it  is  doubtful  whether  or  not  all  these  statements  relate  to 
man.  With  the  introduction  of  modem  imbedding  and  section  technique  they  have, 
however,  been  forgotten,  for  it  must  be  that  the  interciliary  fibers  of  Czermak  belong 
in  this  category.  I  have  again  demonstrated  their  regular  presence  and  have  more 
accurately  described  their  situation  and  their  course  (184). 

According  to  Graf  Spee  (210),  two  additional  girdles  of  circular  fibers  are  found, 
one  between  the  ora  serrata  and  the  corona  clliaris  and  one  at  the  border  of  the  patellar 
fossa. 


The  zonula  throughout  consists  of  structureless,  non-nucleated  fibers, 
clear  as  glass,  which  are  stiff  in  comparison  with  other  fibers,  i.e.,  they 
show  no  tendency  to  wavy  undulation  when  they  are  relaxed,  but 
rather  show  angular  nickings. 

The  finest  fibers  are  encountered  in  the  posterior  border  of  the  zonula 
and  everywhere  along  the  inner  surface  of  the  ciliary  body.  They  are 
much  too  fine  to  be  accurately  measured ;  they  are,  however,  considerably 
thicker  than  the  vitreous  fibrillae,  and  in  sections  are  especially  differen- 
tiated by  their  taut,  straight  course.  The  largest  zonula  fibers  have  a 
thickness  of  30  to  40  mu;  all  possible  transitions  lie  between. 

The  cross-section  of  a  large  fiber  (PI.  VII,  5,  Z)  shows  an  irregular 
form  from  elevations  and  constrictions,  and  these  are  to  be  noted  as 
fine  longitudinal  striae  in  the  longitudinal  view.  This  would  show, 
apparently,  that  we  have  in  these  larger  fibers  bundles  of  finer  ones, 
but  the  constituent  parts  of  such  a  bundle  are  so  thoroughly  fused  into 
one  another  that  neither  the  lines  of  separation  nor  the  special  cement 


THE  ZONULA  CILIARIS  i6i 

substance  can  be  made  out.  A  similar  fusion  is  also  found  where  the 
zonula  fibers  cross.  The  compound  nature  of  the  large  fibers  is  shown, 
too,  by  the  fact  that  it  breaks  up  into  fine  fibers  at  the  ends. 

These  branches  are  either  brush-form,  i.e.,  the  fine  fibers  diverge  in  all 
directions,  or  fan-like,  i.e.,  the  fibers  diverge  out  into  one  plane  from  a 
given  point,  or  like  a  feather,  i.e.,  the  fine  fibers  gradually  divide  off 
from  the  large  one.  The  purest  of  the  two  first  tjpes  are  found  at  the 
inner  (axial)  ends,  i.e.,  at  the  insertion  into  the  lens  capsule  and  the 
vitreous;  the  third  t}-pe  appears  along  the  inner  surface  of  the  orhicithis 
ciliaris. 

If,  indeed,  the  large  zonula  fibers  do  not  with  certainty  form  a  histo- 
logic unit  it  is  certainly  not  possible  to  distinguish  between  elementary 
fibrillae  and  compound  fibers  in  an  unexceptionable  way;  one  does  not 
know  where  to  draw  the  line  among  the  multitudinous  transitions  in 
size  and  form  on  a  cross-section. 

With  respect  to  the  tinctorial  relations  the  zonula  fibers  agree  w'ith  the 
glass  membranes  (cuticular  formations).  It  follows  that  a  certain  affinity 
for  orcein  is  present.  It  is  a  striking  fact  that  they  take  the  Weigert 
neuroglia  stain  (Agababow,  5).  Yet  it  is  going  too  far  to  deduce  from  this 
a  relationship  to  neuroglia,  for,  in  my  preparations  at  least,  other  elements, 
which  have  not  the  least  relationship  to  neuroglia,  take  this  stain  as  well. 

The  typical  zonula  fibers  unite  with  cuticular  structures  at  both  ends, 
in  my  opinion,  with  the  lens  capsule  on  the  one  side  (cf.  chap,  xiii),  and 
with  the  limitans  interna  ciliaris  on  the  other  side. 

With  respect  to  the  first  point,  i.e.,  the  union  with  the  lens,  not  the  slightest  differ- 
ence of  opinion  exists.  So  far  as  the  second  point  is  concerned,  i.e.,  the  union  with 
the  limitans  interna  ciliaris,  there  are  a  number  of  authors  who  dilTer  in  their  \iews. 

Schoen  (192)  thinks  one  zonula  fibrilla  proceeds  out  of  each  ciliary  epithelial  cell. 
According  to  Terrien  (221),  they  come  out  between  these  cells  and  take  their  origin 
from  the  glass  membrane  of  the  ciliary  body  (our  cuticular  lamella,  p.  117).  Wolfrum 
(242),  finally,  claims  to  have  followed  the  finest  extensions  of  the  zonula  fibrillae  through 
the  protoplasm  of  the  epithelial  cells  up  to  the  cement  ridge  between  the  ciliary-  epithe- 
lium and  the  pigment  epithelium. 

It  is  conceivable  that  all  of  the  authors  who  consider  that  the  zonula  fibers  are 
given  off  from  the  ciliary  epithelium  do  not  admit  the  existence  of  a  limitans  interna 
ciliaris,  at  least  in  those  places  where  the  zonula  fibers  are  given  off. 

Special  relations  exist  between  the  ridges  which  the  limitans  interna 
ciliaris  forms  in  the  anterior  part  of  the  orbiculus  and  the  straight-  and 
backward-coursing  zonula  fibers.  On  transverse  section  (PI.  VII,  5), 
one  sees  the  cross-sections  of  new  fine  zonula  fibers  here  and  there  between 
the  closely  apposed  leaves  of  such  a  ridge,  and  in  other  places  small  rows 
of  zonula  fibers  lie  just  over  the  ridges,  out  of  which  they  apparently 


i62  ANATOMY  AND  illSTOLOGV  OF  THE  HUMAN  EYEBALL 

come,  as  shown  by  Lheir  oblique  course.  In  these  places  the  zonula  fibers 
certainly  do  not  press  into  the  protoplasm  of  the  ciliary  epithelium. 
They  remain  keyed  in  between  the  leaves  of  the  ridge  which  incloses  them 
in  a  fold. 

It  appears  to  me  highly  probable,  although  I  cannot  yet  prove  it, 
that  these  fibers  pass  over  the  ridges,  but  do  not  end  in  them,  i.e.,  that 
they  are  fibers  which  come  from  behind  and  appear  on  the  ridges,  describe 
a  fiat  bow,  and  come  out  in  front  again  as  straight-coursing  fibers. 

Nothing  can  be  accurately  stated  concerning  the  union  of  the  zonula 
fibers  with  the  tissue  of  the  vitreous.  One  can  follow  such  a  zonula  fiber 
only  a  short  distance  into  the  tissue  of  the  vitreous,  then  it  disappears; 
whether  or  not  it  goes  over  into  vitreous  fibrillae  remains  unsettled. 

The  cells  occasionally  seen  upon  the  zonula  fibers  have  been  subjected 
to  very  different  interpretations.  According  to  my  observations,  one 
finds  the  same  cells  on  the  outer  surface  of  the  vitreous  base  as  along  the 
posterior  border  layer.  Occasionally  such  cells  are  displaced  still  farther 
forward  onto  the  zonula  fibers.  They  are  all  wandering  cells.  The  folds 
and  rolls  of  the  ciliary  epithelium  may  give  rise  to  confusion,  as  for 
instance,  when,  as  in  older  people,  the  membrana  limitans  interna  ciliaris 
projects  forward  between  the  ridges  and  the  meridional  section  cuts  them 
longitudinally.  Perhaps  the  cell-layer  between  the  ciliary  epithelium  and 
the  zonula  described  by  Graf  Spee  (210)  belongs  in  this  category.  No 
cells — not  even  w'andering  cells — are  found  in  the  free  part  of  the  zonula. 

I  have  not  been  able  to  convince  myself  of  a  direct  connection  between 
the  zonula  fibers  and  the  border  of  the  retina  or  the  supporting  tissue  of 
the  retina,  as  contended  for  by  Schoen  (192). 


CHAPTER  Xin.     THE  LENS  (LENS  CRYSTALLINA) 

The  lens  has  the  form  of  a  biconcave  lens  with  unequal  sides  and 
rounded  border;  accordingly,  one  differentiates  an  anterior  and  a  posterior 
lens  surface,  and  a  lens  border  or  equator  leiitis.  The  center  of  the 
anterior  surface  is  designated  as  the  anterior  lens  pole,  that  of  the  pos- 
terior surface  as  the  posterior  lens  pole,  and  the  line  of  union  between 
the  two,  the  axis  of  the  lens. 

Looked  at  from  in  front  or  behind,  the  lens  has  a  circular  form  and  a 
diameter  of  about  9  mm'  (PL  II,  i).  This  dimension  is  the  frontal  or 
equatorial  diameter.     The  lens  border  is  not  entirely  smooth,  but  pro- 


■  .\11  of  the  meisurem^nts  and  statements,  where  not  expressly  otherwise  stated,  relate  to  a  middle 
age  of  life  and  with  the  lens  focused  for  distance.  Concerning  the  influence  of  the  age  of  life  upon  the 
lens,  see  chap,  xviii. 


THE  LENS  163 

vided  with  rounded  or  dentate  prominences  corresponding  to  the  ciliary 
valleys  and  apparently  arising  from  the  pull  of  the  zonula  fibers.  It  is 
not  a  form  of  hardening  effect,  for  it  can  be  seen  in  the  living  lens  as 
well  (Magnus,  146;  Topolanski,  223).  According  to  Hess  (96),  they  are 
more  marked  in  atropinized  eyes  than  in  eserinized  eyes. 

The  form  of  the  anterior  surface  varies  only  a  little  from  that  of  the 
surface  of  a  sphere.  At  its  vertex  the  radius  of  curvature  is  8.4  to 
13.8  mm — an  average  of  10 .  64  mm — according  to  the  numerous  measure- 
ments of  Tscherning's  (228)  students,  Auerbach,  Saunte,  Maklakoff, 
and  others;  and,  according  to  the  latest  measurements  of  Zeeman  (244), 
the  average  in  emmetropes  is  1 1 .  05  mm. 

The  form  of  the  posterior  surface  is  that  of  a  paraboloid  with  a  vertex 
curvature  of  4.6  to  7.5  mm,  an  average  of  5 .  98  mm.  A  flat  circular 
furrow  (concavity  forward)  has  been  found  by  several  authors  in  the 
peripheral  zone  of  the  posterior  surface;  Zeeman  (243)  has  found  it  in 
the  living  by  the  doubling  of  the  image  reflected  by  the  posterior  lens 
surface  at  a  distance  of  3  . 5  mm  from  the  optic  axis,  and  von  Pflugk  (172) 
in  the  newborn  after  fixation  of  the  form  of  the  lens  by  his  freezing  method. 

From  ophthalmometric  measurements,  the  thickness  or  sagittal 
diameter  of  the  lens  is  2  . 9  to  5 .  i  mm,  an  average  of  3  .  7  mm  (Tscherning) , 
an  average  of  3.76  mm  in  emmetropes,  according  to  Zeeman.  In  ana- 
tomic study,  however,  the  thickness  of  the  lens  will  be  found  greater, 
and  according  to  the  old  measurements  of  Pourfour  du  Petit,  contributed 
by  Tscherning,  an  average  of  4.7  mm — according  to  Krause,  4.9  mm. 

Meridional  asymmetry  has  not  heretofore  been  certainly  proven  for 
the  lens,  yet  many  things  speak  for  this  and,  too,  for  the  fact,  that  the  lens 
possesses  a  slight  degree  of  meridional  asymmetry  or  in  any  case  acquires 
it,  in  later  life. 

A  correct  conception  of  the  form  and  size  of  the  lens  can  only  be  obtained  by  a  study 
of  fresh  material.  All  of  the  fi.xation  and  hardening  means  lead  to  a  considerable 
shrinking  and  change  of  form  in  the  lens.  The  equatorial  diameter  shortens  about 
I  mm,  and  the  anterior  surface  takes  on  a  stronger  curvature. 

But  the  fresh  lens  also  changes  its  form  markedly,  and  the  curvature  of  the  surfaces 
is  lost  as  soon  as  it  is  separated  from  its  connections.  The  task  of  fixing  the  natural 
form,  of  the  lens,  i.e.,  that  present  in  life,  is  an  especially  difficult  one;  von  Pflugk 
thought  he  had  solved  this  with  his  freezing  method;  according  to  Hess  (99),  this  suc- 
ceeds only  after  previous  fixation  with  formalin. 

The  lens  is  completely  clear  and  transparent  in  the  fresh  state,  but, 
according  to  Hess  (97),  it  is  never  entirely  without  color,  and  even  in  }-outh 
has  a  tinge  of  yellow  which  always  becomes  more  marked  with  age. 

The  union  of  the  lens  with  its  environment  is  effected  by  means  of 
two  structures:    the  zonula  ciliaris  unites  it  with  the  ciliary  bodv,  the 


i64  ANATOMY  AND  HISTOLOGY  OF  THE  HUMAN  EYEBALL 

ligamciitum  hyaloideo-capsidarc  wilh  the  \ilreous  body.  The  first  union 
is  by  far  the  more  important  one,  for  even  in  complete  relaxation  of  the 
zonula  (and  still  more  after  it  is  separated)  the  lens  loses  its  fixed  position 
and  sinks  from  gravity.  Complete  dislocation  (luxation)  of  the  lens  is 
only  possible,  however,  when  the  ligamentum  hyaloideo-capsidarc  has  also 
been  torn. 


In  histologic  respects,  three  different  constituent  parts  of  the  lens 
can  be  demonstrated.  From  without  inward,  they  are:  the  lens  capsule, 
the  lens  epithelium,  and  the  lens  substance.  The  latter  is  a  product  of 
the  epithelium,  and,  thanks  to  the  never  completed  growth  of  the  lens, 
one  finds  the  lens  substance  in  process  of  formation  at  every  age  of  life. 
The  histologic  peculiarities  of  this  will  be  described  under  the  heading 
of  "epithelial  border." 

I.      THE   LENS   CAPSULE 
(PI.  LX,  3,  4,  A') 

This  is  a  typical  glass  membrane,  i.e.,  a  structureless,  highly  refracting, 
very  firm  elastic  membrane,  highly  resistant  to  chemical  and  pathologic 
influence,  whose  wound  borders  have  a  marked  tendency  to  roll  outward. 
A  lamellar  composition  is  not  ordinarily  seen,  yet  a  thin  superficial  lamella 
often  appears  in  the  region  of  the  lens  equator;  it  carries  the  last  exten- 
sions of  the  zonula  fibers  and  has  been  called  the  zonular  lamella  by 
Berger  (20),  the  pericapsular  membrane  by  Retzius  (iSo). 

The  lens  capsule  forms  a  closed  hull  about  the  lens.  Solely  upon 
practical  grounds,  such  as  those  of  operative  technique,  one  differentiates 
an  anterior  capsule,  i.e.,  that  part  drawn  over  the  anterior  surface,  and  a 
posterior  capsule,  the  covering  of  the  posterior  surface. 

The  thickness  of  the  lens  capsule  shows  wide  variation  among  indi- 
viduals and  at  different  ages,  for  it  increases  appreciably  in  thickness 
with  age.  But  the  thickness  of  the  capsule  also  varies  in  different  zones 
in  the  same  lens.  The  table  on  p.  165  gives  a  survey  of  the  measurements 
of  this  variation. 

In  general,  the  anterior  capsule  and  the  region  of  the  equator  is  thicker 
than  the  posterior  capsule.  The  minimum  thickness  lies  at  the  posterior 
pole  under  all  circumstances. 

Since  this  is  constant  and  easy  to  be  made  out,  it  can  be  made  use  of  for  the  orienta- 
tion of  the  surfaces  of  the  lens,  e.g.,  anomalies  in  the  position  or  situation  of  the  lens. 

The  thickest  portions  of  the  capsule  form  two  zones  concentric  with 
the  equator,  one  on  the  anterior,  one  on  the  posterior  surface.  The  zone 
of  maximum  thickness  of  the  anterior  surface  lies  about  3  mm  from  the 


THE  LENS 


I6S 


anterior  pole  or  i  mm  inside  (axial  to)  the  insertion  girdle  of  the  anterior 
zonula  fibers.  The  zone  of  the  maximum  thickness  of  the  posterior  fibers 
lies  still  farther  peripheralward,  somewhat  inward  from  the  posterior 
zonular  insertions  and  the  ligamentiim  hyaloideo-capsulare.  Not  infre- 
quently this  maximum  exceeds  that  of  the  anterior,  as  for  instance,  in 
the  child's  eye  (see  No.  i  of  the  table). 

The  thickness  of  the  equatorial  zone  varies  much;  the  measurement 
given  in  the  table  is  taken  from  immediately  behind  the  epithelial  border, 
where  a  relative  minimum  lies.  In  front  of  and  behind  this  place  the 
capsule  is  somewhat  thicker. 


Age 

Thickness  of  the  Lens  Capsule  in  Md 

No. 

Anterior  Pole 

Maximum  of 

tile  Anterior 

Surface 

Equator 

Maximum  of 

the  Posterior 

Surface 

Posterior  Pole 

14  days 
2 . s  years 

7 

9 
IS 
19 
23 
26 
32 
35 

36        " 
40        " 
41 

48        " 
Si 

56        " 
71 

6 
8 
8 
8 
9 
12 
II 
10 
12 
14 
9 
16 
II 
II 
14 
18 
14 

s 

12 
13 

15 
14 
23 
18 
18 
16 
21 
21 
22 
18 
22 
25 
23 
21 

3 

7 

g 

8 

14 
17 
14 
10 
16 
17 
16 
16 
18 
15 
16 
14 
9 

18 
18 
17 
22 
23 
26 
21 
17 
21 
23 
22 
18 
23 
28 
23 
16 

9 

2-5 

3 
3 
3 
3 

2-3 

4 

3-4 

3 

3 

3-4 

3 

3 

2-3 

6          .... 

8 

i6 

The  greatest  interest  attaches  to  the  zones  of  the  maxima.  The 
anterior  maximum  corresponds  to  that  place  in  which  Tscherning  (227) 
found  a  flattening  of  the  anterior  lens  surface  in  accommodation;  the 
posterior  maximum  corresponds  to  that  place  in  which  Zeeman  (243)  and 
von  Pflugk  (172)  have  seen  concavities.  Both  maxima  lie  somewhat 
axial  to  the  insertions  of  the  zonula  fibers,  but  they  are  not  brought 
about  by  these,  for  at  the  very  places  of  insertion  the  thickness  of  the 
capsule  is  less. 

The  insertion  of  the  zonula  fibers  follows  a  zone  concentric  with  the 
equator  and  inclosing  it.  The  anterior  zonula  fibers  reach  much  farther 
over  onto  the  anterior  than  do  the  posterior  zonula  fibers  onto  the  posterior 
surface.  As  a  result,  the  equator  does  not  lie  in  the  center  of  this  zone 
but  it  divides  it  somewhat  in  the  relation  of  3  to  2  (PI.  I).  Measured 
along  the  surface,  the  whole  insertion  zone  is  about  2  mm  broad;  measured 
along  a  chord,  1.3   to  i .  9  mm,  depending  upon  the  degree  of  rounding  of 


i66  ANATOMY  AND  HISTOLOGY  OF  THK  HUMAN  EYEBALL 

Ihe  lens  border;  its  posterior  border  coincides  almost  exactly  witli  the 
insertion  of  the  llgamentum  liyaloidco-capsulare  into  the  lens  capsule. 

The  angle  at  which  the  zonula  fibers  are  inserted  into  the  lens  cai^sule 
varies  between  o°  and  90°,  for  the  most  anterior  and  posterior  fibers 
approach  the  lens  capsule  in  a  tangential  direction,  the  middle  ones  at 
right  angles;  the  fibers  lying  in  between  show  an  angle  of  insertion  increas- 
ing as  they  approach  the  equator.  The  zonula  fibers  are  broken  up  first 
upon  the  lens  capsule  into  the  last  finest  extensions  (in  so  far  as  this  has 
not  already  occurred  in  the  free  portion),  and  then  continue  a  stretch 
farther  on  over  the  capsule  in  a  meridional  direction  (PI.  IX,  i).  Espe- 
cially is  this  true  of  the  tangentially  inserted  fibers,  i.e.,  those  lying  at 
the  borders  of  the  insertion  territory  (they  are  inserted  with  band-like 
expansions) ;  their  expanse  reaches  about  o .  4  mm  farther  forward  beyond 
the  margins  of  the  insertion  zone. 

The  middle  zonula  fibers  {qZ)  divide  previously  into  brushes  of  finest 
fibers,  which  course  without  further  subdivision  to  the  surface  of  the 
capsule  and  then  go  partly  forward,  partly  backward. 

The  extensions  of  the  zonula  fibers  remain  on  the  surface  of  the  lens 
capsule  throughout,  as  can  best  be  seen  in  equatorial  sections;  zonula 
fiber  extensions,  which  show  varying  size  upon  cross-section,  lie  only 
upon  the  outer  surface  of  the  lens  capsule.  On  surface  view  they  show  a 
meridional  striation,  which  is  not  wholly  uniform,  i.e.,  it  is  made  up  of 
larger  and  smaller  elements ;  this  is  due  to  the  fact  that  the  border  fibers 
extend  beyond  the  limits  of  the  insertion  zone.  In  general,  they  extend 
over  a  girdle  2.7  mm  broad  (measured  along  the  surface).  The  borders 
of  this  girdle  are  circles  concentric  with  the  ecjuator  (parallel  circles); 
this  is  especially  plain  on  the  anterior  lens  surface  on  account  of  the  uni- 
form length  of  the  zonula  fiber  extensions  {vZ).  When,  however,  one 
observes  the  actual  points  of  insertion  of  the  zonula  fibers,  and,  therefore, 
excludes  the  extensions  coursing  over  the  surface  of  the  lens  capsule,  there 
appears  a  girdle  of  measurable  breadth  and  not  mathematical  lines,  as 
stated  by  Schoen  (160),  although  marked  individual  variations  are 
shown;  if  one  takes  into  consideration  only  the  fibers  lying  on  the  surface 
of  the  whole  fiber  mass  the  insertions  form  very  wavy  lines,  especially  on 
the  posterior  surface  {hZ).  This  alternating  projection  and  retraction  of 
the  most  superficial  fibers  is  probably  the  reason  why  the  older  anatomists 
conceived  of  the  zonula  as  a  ruffled,  folded  membrane. 

On  the  inner  surface  of  the  posterior  capsule  one  not  infrequently  sees 
a  system  of  lines  which  inclose  polygonal  cell-like  fields;  these  are  the 
impressions  of  the  bases  of  the  lens  fibers  in  a  thin  layer  of  coagulated 
fluid,  which  is  secreted  as  a  post-mortal  appearance  between  the  lens 
substance  and  the  lens  capsule. 


THE  LENS  167 

2.      THE    LENS    EPITHELIUM 

This  extends  beneath  the  lens  capsule  over  the  whole  anterior  surface 
of  the  lens  up  to  the  equator  (and  sometimes  a  short  distance  beyond  it). 

This  epithelium  is  disposed  in  two  biologically  different  zones:  the 
one,  covering  over  the  anterior  lens  surface,  has  no  relation  to  the  forma- 
tion of  lens  fibers  and  even  under  pathologic  conditions  is  not  capable  of 
forming  such  (its  pathologic  product  is  the  capsular  cataract) ;  yet  it  plays 
a  significant  part  in  the  nutrition  of  the  lens  and  its  insult  leads  to  a  cloud- 
ing of  the  lens  substance.  On  the  other  hand,  the  narrow  seam  of  the 
epithelium,  which  lies  at  the  equator  of  the  lens,  the  epithelial  border, 
furnishes  the  lens  fibers  and  thereby  cares  for  the  growth  of  the  lens; 
this  growth  continues  throughout  the  whole  of  life,  although,  indeed,  with 
gradually  decreasing  intensity. 

a)  The  Epithelium  of  the  Anterior  Lens  Surface 

In  the  neighborhood  of  the  anterior  pole  these  cells  are  11  to  17  niu 
broad  and  5  to  8  mu  high ;  the  nuclei  are  round  in  surface  view  and  have  a 
diameter  of  7  mu  (PL  IX,  2) ;  in  cross-section  they  are  elliptical.  The 
arrangement  of  the  cells  is  not  a  regular  one  and  as  a  result  the  form  of 
the  ceUs  is  not  regular.  In  a  fresh  state  the  cell  borders  appear  as  fine 
sharp  lines  whose  position  and  course  change  with  varying  focus,  i.e.,  the 
lateral  separating  surfaces  do  not  lie  at  right  angles  to  the  capsule  but  are 
bowed.  This  comes  out  more  clearly  when  the  epithelium  is  impregnated 
with  silver  (Brabaschew,  15),  and  then  one  sees  two  systems  of  separating 
lines  which  do  not  coincide.  Spaces  are  not  infrequently  found  between 
the  cells,  and  the  portions  connected  are  drawn  out  into  bridges  between 
the  cells,  and  the  individual  cells  take  on  a  more  or  less  stellate  form 
(Hosch,  106).  This  latter  appearance  is  apparently  a  result  of  the  shrink- 
ing of  the  cell,  probably  coming  about  through  various  influences;  pos- 
sibly, too,  it  occurs  during  life. 

Farther  toward  the  periphery  the  arrangement  remains  the  same,  but 
the  cells  are  notably  smaller  (8  to  12  mu  in  surface  expanse)  and  higher 
(9  to  15  mu),  i.e.,  the  cells  approach  the  cylindrical  form,  and  the  nuclei 
back  up  toward  the  inner  end  of  the  cell  and  become  spherical  (PL  IX, 
3 — the  uppermost  cells  of  the  epithelium). 

j3)  The  Epithelial  Border  and  the  Lens  Vortex  (Becker,  181 :   the  Formation  of  New  Lens  Fibers 

At  the  epithelial  border  the  heretofore  irregularly  arranged  cells  are 
arranged  in  meridional  rows.  It  is  the  service  of  Rabl  (175)  to  have 
recognized  this  linear  arrangement  as  a  tj^Dical  appearance  in  the  mam- 
malian lens  and  to  have  brought  out  its  fundamental  significance  for 


i68  ANATOMY  AND  HISTOLOGY  OF  THE  HUMAN  EYEBALL 

the  structure  of  the  lens  substance.  In  man,  especially  in  the  adult, 
these  rows  are  very  short  and  indistinct,  i)robably  because  the  cells  grow 
out  so  soon  from  the  lens  fibers. 

When  one  studies  an  exactly  meridional  section  of  this  region  (PI. 
IX,  3),  a  broadening  of  those  parts  abutting  upon  the  capsule  is  seen 
to  be  the  first  change  in  the  epithelial  cells.  The  first  of  these  cells 
(cell  6  from  above)  takes  on,  thereby,  the  form  of  a  trunkated  pyramid 
and  the  cells  following  become  more  and  more  oblicjue.  The  obliquity, 
however,  affects  only  the  part  of  the  cells  turned  toward  the  capsule  at 
first;  the  inner  parts  of  the  cells  (in  which  the  nuclei  lie)  retain  their 
form.  These  cells  (11  and  12  from  above)  also  take  on  an  oblique  direc- 
tion; the  nucleus  becomes  larger  and  oval  on  cross-section.  Farther 
along  the  inner  part  of  the  cell  elongates  more  and  more,  its  obliquity 
increases  and  soon  exceeds  that  of  the  outer  part,  so  that  the  whole  cell 
accjuires  a  bowing  (concavity  forward  and  toward  the  capsule). 

Under  further  elongation  the  cell,  or  rather  the  young  lens  fiber, 
interposes  its  inner  (now  its  anterior)  end  between  the  epithelium  and 
the  older  part  of  the  lens  substance,  while  the  outer  end  is  constantly 
pushed  backward  by  the  subsequently  growing  cell.  The  nucleus  thereby 
becomes  displaced  farther  forward,  yet  in  a  degree  varying  a  great  deal 
among  the  different  cells. 

The  nuclei  of  the  epithelium,  along  with  those  of  the  young  lens 
fibers,  form  a  very  characteristic  figure  on  meridional  section,  the  pecu- 
liarities of  which  are  to  be  recognized  even  by  moderate  magnification; 
the  older  authors  called  it  the  nuclear  zone,  Becker,  the  nuclear  bow. 
In  the  epithelium  proper  of  the  anterior  lens  capsule  the  nuclear  row  runs 
parallel  to  the  capsule;  in  the  neighborhood  of  the  epithelial  border  it  is 
somewhat  removed  from  the  capsule  and  at  the  very  epithelial  border 
bows  about  to  the  front  in  a  sharp  curve  or  angle,  but  at  the  same  time 
the  nuclei  fall  more  and  more  into  this  order. 

If  one  calls  that  end  of  an  epithelial  cell  turned  toward  the  surface  the 
free  end  (as  does  Rabl,  175),  or  the  head  of  the  cell  (in  analogy  to  the 
basal  cells  of  the  corneal  epithelium),  the  end  turned  toward  the  meso- 
derm the  basis  on  the  other  hand,  and  the  line  of  union  between  the  two 
the  main  axis,  these  poles  of  the  lens  epithelium  are  oriented  according 
to  the  development  as  follows:  the  bases  of  the  cells  lie  on  the  lens  cap- 
sule, the  heads  of  the  cells  look  toward  the  lens  substance,  and  the  main 
axes  are  at  right  angles  to  the  capsule. 

In  the  transition  into  a  lens  fiber  the  epithelial  cell  grows  in  the 
direction  of  the  main  axis,  but  at  the  same  time  this  undergoes  a  rotation 
of  90°  with  the   inner   end   forward.     The  bases  of  all  the  lens  fibers. 


THE  LENS  169 

therefore,  come  to  lie  in  the  posterior  part,  the  heads  in  the  anterior  part 
of  the  lens. 

By  the  rotation  of  the  individual  elements  during  development  there 
arises  that  peculiar  figure  seen  at  the  epithelial  border  in  a  meridional 
section  called  by  Becker  (18)  the  lens  vortex. 

The  expression  lens  vortex  was  used  by  the  earlier  authors  and  by  Rabl,  too,  to 
indicate  those  tigures  which  arise  through  the  meeting  of  the  fibers  in  the  lens  stars. 
Schwalbe  called  this  figure  at  the  epithelial  border  the  border  vortex,  for  this  reason. 

The  lens  vortex  forms  an  inward-projecting  roll  as  a  result  of  the 
pressing  together  of  the  heads  of  the  cells,  and  the  youngest  lens  fibers, 
therefore,  acquire  a  concavity  forward.  Because  now  each  fiber  is  thinner 
over  this  roll  than  in  front  of  or  behind  it,  the  concavity  gradually  flattens 
out  inward  and  finally  goes  over  into  a  definite  convexity  (corresponding 
to  the  curvature  of  the  equatorial  portions  of  the  lens).  In  only  one 
place,  at  the  nucleus,  is  the  young  fiber  thicker.  But  these  areas  are  not 
superimposed  and  in  this  way  they  are  smoothed  out. 

In  a  strictly  meridional  section  there  is  a  meridional  row  of  epithelial 
cells  with  lens  fibers  coming  out  of  these,  and  when  one  follows  the  direc- 
tion of  the  epithelial  cells  from  before  backward  (the  lens  fibers  in  the 
direction  from  without  inward),  one  surveys  at  the  same  time  a  whole 
row  of  different  developmental  stages,  of  which  the  differences  in  respect 
to  age  are  nearly  uniform.  The  temporal  interval  of  the  developmental 
process  thus  corresponds  to  a  juxtaposition  in  space,  and  this  series  has, 
in  general,  the  character  of  an  arithmetical  progression. 

The  more  vigorous  the  growth  of  the  lens,  e.g.,  in  children,  the  slighter 
are  the  differences  in  age  in  this  series;  the  more  developmental  stages, 
one  after  another,  one  has  before  him,  the  more  gradual  is  the  transition 
of  the  epithelium  into  the  lens  substance,  the  more  round  and  long  is 
the  nuclear  bow.  The  more  indolent  the  growth,  e.g.,  in  old  people, 
the  greater  are  the  differences  in  age  in  a  series,  the  fewer  the  develop- 
mental stages  visible  at  one  time,  the  sharper  is  the  demarkation  of  the 
epithelium  from  the  lens  substance,  the  shorter  and  more  angular  is  the 
nuclear  bow. 

That  which  proceeds  from  a  meridional  row  of  epithelial  cells  remains 
constant  in  the  meridian.  The  complete  lens  fibers  form  rows,  there- 
fore, even  as  do  the  epithelial  cells,  and  because  the  individual  elements 
of  these  rows  have  significantly  elongated  in  the  direction  of  their  main 
axes,  each  such  row  has  two  dimensions  :  it  becomes  a  meridional  or  radial 
lamella  (Rabl,  175),  the  most  important  textural  element  of  the  lens 
structure. 


I70  ANATOMY  AND  HISTOLOGY  OF  THE  HUMAN  EYEBALL 

3.      THE   LENS   SUBSTANCE 

In  middle  life  the  outer  layers  of  the  lens  substance  are  very  soft  and 
without  color  (cortex),  the  deeper  layers  are  appreciably  harder  and  more 
or  less  of  a  yellow  color  (lens  nucleus) .  Corresponding  to  the  consistence, 
the  index  of  refraction  also  changes;  according  to  Halben  (85),  it  varies 
between  1.36  (cortex)  and  1.4452  (nucleus).  Heine  (89)  gives  the 
nucleus  a  maximum  index  of  1.41. 

As  Halben  has  shown  by  his  differential  refractometer,  the  transition 
of  the  cortex  into  the  nucleus  is  completed  to  a  certain  depth  pretty 
suddenly,  i.e.,  there  is  a  rapid  increase  of  the  index  of  refraction.  A 
transition,  indeed,  does  exist,  but  this  is  not  as  gradual  as  Matthiessen 
(148)  had  earlier  conceived  it  to  be.  The  thickness  of  the  transition  zone 
is  so  slight  that  the  image  reflected  by  it  is  quite  plain,  as  Hess  (97)  and 
his  students  have  shown. 

For  example,  when  one  studies  the  images  of  the  lens  with  an  intense 
linear  light,  one  perceives  a  dimmer  and  weaker  picture,  it  is  true,  but 
yet  another,  alongside  of  the  two  well-known  Purkinje-Sanson  images 
of  the  anterior  and  posterior  surfaces  (which,  according  to  Hess,  should 
rather  be  spoken  of  as  the  anterior  and  posterior  cortex  images) ;  these 
move  in  the  same  direction  as  the  neighboring  cortex  images  and  are 
directed  the  same  as  these.  They  are  thrown  by  the  anterior  and  pos- 
terior nuclear  borders  and  carry  the  names  anterior  and  posterior  nuclear 
images.  According  to  Freytag  (60),  the  anterior  becomes  constant  after 
the  twenty-fourth,  the  posterior  after  the  thirty-first  year  of  life.  With 
increasing  age,  these  little  images  increase  in  luminosity,  i.e.,  the  difference 
in  the  index  of  refraction  becomes  greater. 

With  the  exception  of  the  central  portions,  the  whole  lens  substance 
is  made  up  of  the  characteristic  radial  lamellae  of  Rabl,  described  above 
(PI.  IX,  4).  The  number  of  the  lamellae  on  the  surface  in  the  adult 
can  be  considered  as  2,100  to  2,300.  With  increasing  depth  the  number 
of  lamellae  decreases,  partly  because  the  individual  lamellae  sharpen  out, 
partly  because  neighboring  lamellae  fuse.  In  this  way  the  lamellae 
become  more  and  more  indistinct  the  nearer  one  approaches  the  center 
(PI.  IX,  5);  in  the  center  the  arrangement  of  the  sagittally  coursing 
fibers  is  irregular  (Rabl's  central  fibers).  The  human  lens  is,  in  general, 
characterized  by  slight  regularity  in  the  matter  of  formation  of  the 
lamellae. 

Since  the  formation  of  new  lens  fibers  progresses  about  the  whole  cir- 
cumference of  the  lens  equator  and  a  new  fiber  is  started  at  about  the  same 
time  in  each  radial  lamella,  the  lens  substance  acquires  a  stratification 
conditioned  by  the  difference  in  the  age  of  the  fibers.     By  maceration,  e.g., 


THE  LENS  171 

cooking,  these  layers  or  lamellar-complexes  can  be  separated  from  one 
another :   the  lens  can  be  split  up  into  leaves  parallel  to  its  surface. 

Within  such  a  layer  the  fibers  are  arranged  as  follows :  The  fibers  are 
strictly  meridional  only  in  the  neighborhood  of  the  equator  and  only  here 
do  they  form  a  uniform  layer.  As  soon  as  one  comes  upon  the  surfaces 
of  the  lens  in  following  out  the  fibers,  one  perceives  a  division  into  sectors, 
and  the  lines  of  separation  (sutures)  of  these  sectors  form  a  stellate 
figure  on  each  lens  surface  with  the  center  at  the  pole  (the  so-called  lens 
star;  PI.  II,  i). 

The  lens  star  is  indicated  in  a  normal  lens,  even  during  life,  on  lateral 
illumination,  and  often  comes  out  much  more  plainly  in  incipient  cata- 
racts. In  the  anatomic  preparation,  too,  the  cadaverous  appearances, 
or  the  effect  of  fixation  fluids,  often  brings  out  the  lens  stars;  at  other 
times  one  can  make  them  visible  by  a  precipitation  of  silver  along  the 
cement  lines. 

The  lens  star  is  not  a  regular  figure.  In  the  adult  one  can  usually 
count  9  to  12  rays,  the  intervals  between  which  are  unequal;  moreover, 
the  points  of  union  between  neighboring  rays  and  the  pole  do  not 
coincide  exactly,  so  that  at  the  pole  itself  a  figure  with  fewer  arms  arises, 
but  I  have  never  found  the  star  figure  as  irregular  as  Fridenberg  (62) 
draws  it. 

In  each  sector  the  lens  fibers  first  course  parallel  to  the  middle  line, 
then  bow  away  from  the  radii  of  the  star,  and  therefore  meet  at  the  star 
ray  in  very  blunt  angles. 

The  two  stars  of  the  lens  (the  anterior  and  the  posterior)  are,  as  a  rule, 
so  oriented  that  the  rays  of  the  one  fall  in  the  interspaces  of  the  other,  for 
which  reason  the  fibers  reaching  to  the  pole  on  the  one  side  cease  at  the 
end  of  a  star  ray  on  the  other. 

In  the  succeeding  layers  the  figure  of  the  lens  star  is  repeated,  yet 
with  increasing  depth  it  becomes  more  and  more  simple,  until  finally  it 
is  reduced  to  a  three-rayed  star,  the  anterior  one  of  which  has  the  form 
of  an  inverted,  and  the  posterior  that  of  an  erect  Y.  One  sees  exactly 
these  same  relations  in  the  lens  star  when  one  recurs  to  an  earlier  stage 
in  life.  For  it  is  a  peculiarity  of  the  lens  that  it  always  grows  by  a  super- 
imposition  of  newer  layers  upon  the  old.  For  this  reason  one  finds  all  the 
peculiarities  which  the  lens  shows  in  early  childhood  or  in  embryonal 
life  at  corresponding  depths  of  the  adult  lens. 

The  elements  of  which  the  lamellae,  as  well  as  the  layers,  are  made 
up  are  the  lens  fibers.  These  are  flatly  compressed,  prismatic,  almost 
band-like  cells  of  considerable  length.  If  there  were  a  complete  regularity 
in  arrangement  each  fiber  would  be  somewhat  longer  than  one-fourth  of 


172  ANATOMY  AND  HISTOLOGY  OF  TIIIC  HUMAN  EYEBALL 

a  lens  meridian;  as  a  result  of  the  alteration  of  the  lens-star,  according 
to  Becker,  the  length  actually  varies  between  7  and  10  mm. 

The  breadth  of  the  lens  fiber,  i.e.,  the  dimension  at  right  angles  to  the 
length  and  parallel  to  the  surface,  amounts  to  8  to  12,  an  average  of  10  mu 
in  the  region  of  the  equator.  Near  the  end  of  the  star  .ray  the  fiber 
gradually  broadens  out  to  double  this  size,  and  at  the  same  time  takes 
on  a  bowing.  Only  those  fibers  reaching  to  the  pole  or  to  the  division- 
places  of  the  star  rays,  or  their  ends,  escape  this  bowing  and  the  broadening 
of  the  ends. 

The  thickness  of  the  fiber,  i.e.,  the  dimensions  at  right  angles  to  the 
surface,  scarcely  amounts  to  2  mu;  only  at  the  point  where  the  nucleus 
lies  does  it  reach  5  mu. 

The  cross-section  of  a  lens  fiber  has  the  form  of  an  elongated  six- 
sided  figure  with  two  long  and  four  short  sides ;  the  long  sides  are  parallel 
to  the  surface,  and  the  fibers  lie  upon  one  another  with  these  sides  apposed 
(PI.  IX,  4).  But  all  these  measurements  and  the  form  on  cross-section 
only  hold  true  for  the  young  fibers,  those  arranged  regularly  in  lamellae. 
The  relations  of  breadth  and  thickness  and  the  form  on  cross-sections 
changes  in  the  depth  of  the  lens  substance  where  the  arrangement 
becomes  irregular  (PI.  IX,  5). 

The  young  fibers,  those  lying  near  the  surface,  have  smooth  borders, 
a  well-staining  oval  nucleus  of  about  12  mu  in  length,  7  mu  in  breadth  and 
4 . 6  mu  in  thickness,  and  a  transparent  body  in  which  one  can  dift'erentiate 
a  firmer  covering  and  tenacious  fluid  content. 

With  increasing  age,  i.e.,  at  a  depth  of  0.15  mm,  the  cell-nucleus 
disappears.  It  first  takes  on  an  almost  spherical  form,  then  shrinks  to  a 
small,  very  intensely-staining  fragment  lying  in  a  cavity  of  the  size  of 
the  original  nucleus.  Finally,  these  last  traces  also  disappear.  In  such 
an  old  lens  fiber  a  fine  serration  appears  along  the  edge,  the  content  of  the 
fiber  thickens,  and  the  fiber  becomes  homogeneous. 

The  union  between  the  lens  fibers  is  effected  by  a  cement  substance, 
more  developed  on  the  narrow  sides  of  the  fibers  than  it  is  on  the  broad 
sides;  as  usual,  this  can  be  brought  out  by  silver  nitrate.  This  same 
substance  also  appears  in  the  neighborhood  of  the  lens  fibers,  i.e.,  in  the 
rays  of  the  lens  star,  yet  a  more  marked  aggregation  of  homogeneous  or 
drop-like  coagulated  substance  signifies  a  cadaverous  appearance  or  an 
artefact  in  these  and  in  other  places. 

According  to  the  above  statements,  the  picture  of  the  lens  substance  has  a  form 
differing  a  great  deal  according  to  the  method  of  preparation. 

The  surface  view  of  the  lens  substance  can  only  be  obtained  by  a  splitting  up  of 
several  layers.  It  shows  the  lens  fibers  as  broad  bands,  the  bowing  toward  the  star  rays, 
the  broadening  into  these,  etc. 


CHAMBERS  OF  EYEBALL  AND  TOPOGRAPHY  OF  THLS  REGION     173 

A  section  through  the  lens  in  the  plane  of  the  equator  cuts  all  the  lens  fibers  cross- 
wise. This  is  the  direction  of  section  which  best  explains  the  texture  of  the  lens  sub- 
stance. In  PL  IX,  4,  5  I  have  depicted  a  part  of  such  an  equatorial  section  through 
the  lens  of  a  two-weeks-old  child ;  at  this  age  the  structure  of  the  lens  is  still  regular  and, 
moreover,  the  central  parts  show  the  fibers  well. 

In  the  superficial  layers  (PI.  IX,  4)  one  sees  the  regularly  developed  Rabl  lamellae; 
they  appear  as  bands  directed  at  right  angles  to  the  surface  with  borders  formed  by 
fine  zig-zag  lines.  The  cross-sections  of  the  fiber  out  of  which  the  bands  are  put 
together,  are  exactly  superimposed,  but  they  are  not  all  of  the  same  breadth,  and  often 
not  of  the  same  thickness.  The  fibers  in  one  lamella  are  displaced  half  of  the  thick- 
ness of  the  fiber  in  the  neighboring  lamella,  whereby  the  six-sided  form  of  the  cross- 
section  arises.  But  even  in  the  youngest  portions  of  the  lens,  irregularities  are  seen 
here  and  there  (lower  part  of  the  figure).  Indeed,  the  farther  one  goes  from  the  capsule, 
the  greater  become  the  irregularities,  and  at  a  depth  of  1.3  mm  (from  the  equator) 
(PI.  IX,  5)  scarcely  any  indication  of  the  lamellae  remains;  the  form  of  the  cross- 
section  of  the  lens  fiber  is,  therefore,  very  irregular. 

The  meridional  section  (PL  IX,  3)  shows  the  longitudinal  section  picture  of  the 
lens  fibers,  i.e.,  narrow,  smooth-bordered  fibers,  which  are  only  a  little  thicker  at  the 
nucleus.  It  shows,  furthermore,  the  lamellation  (stratification)  of  the  lens  structure; 
each  layer  is  thicker  in  the  region  of  the  equator  than  at  the  poles;  as  a  result,  the  radius 
of  curvature  of  the  layers  decreases  with  the  depth  to  a  marked  extent  and  corresponds 
to  the  depth.  The  border  of  the  lens  stains  less  intensely,  and  the  fiber  contours  are, 
therefore,  plainer,  the  nucleus  characterized  by  a  greater  affinity  for  coloring  substances, 
and  an  indistinct  fibrillation.  On  meridional  section,  too,  one  can  at  times  see  the 
cross-section  picture  of  the  lens  substance,  and,  indeed,  in  the  neighborhood  of  the  pole; 
as  soon  as  the  section  is  other  than  exactly  meridional,  it  may  cut  across  the  tapering 
end  of  a  sector  of  lens  fibers. 

Under  the  influence  of  hardening  fluids,  artefacts  very  easily  arise  in  the  lens  sub- 
stance, i.e.,  spaces  filled  with  a  homogeneous  fluid,  or  spheres  of  coagulated  fluid,  or  a 
layer  of  fluid  develops  between  the  capsule  and  the  lens  substance. 


CHAPTER  XIV.  THE  CHAMBERS  OF  THE  EYEBALL  AXD  THE 
TOPOGRAPHY  OF  THIS  REGION 

a)   Tlic  Posterior  Chamber 

The  posterior  chamber  is  bounded  on  the  one  side  by  the  tunica  iiitcrjia, 
and,  indeed,  by  the  ciliary  epithelium  and  the  pigment  epithelium  of  the 
iris,  on  the  other  side  by  the  anterior  border  layer  of  the  vitreous  up  to 
the  ligamentum  hyaloideo-capsiilare  and  from  there  on  by  the  surface  of  the 
lens.  Since  the  pupU-border  of  the  iris  lies  upon  the  lens,  the  posterior 
chamber  has  a  limitation  which  is  neither  absolute  nor  fixed  on  this  side. 
It  is  not  the  former,  because  the  pupil-border  of  the  iris  is  not  grown  to 
the  lens.  Yet  it  appears  from  the  observations  of  Ulbrich  (229)  that  no 
continuous  stream  of  fluid  flows  from  the  posterior  into  the  anterior 
chamber — the  difference  in  pressure  is  only  equalized  from  time  to  time, 


174  ANATOMY  AND  HIS'l'OLOCiY  OF  THE  HUMAN  EYEliAIJ. 

usually  after  a  change  in  the  width  of  the  i>ui)ils.  The  i)().siti()n  of  this 
border  also  changes  as  a  result  of  the  pupil  play. 

In  this  wider  sense,  the  posterior  chamber  is  a  space  of  very  compli- 
cated form,  and  for  the  purposes  of  description,  it  is,  therefore,  advisable 
to  subdivide  it  into  smaller  portions. 

The  most  posterior  (peripheric)  portion  of  the  posterior  chamber  is 
that  narrow  cleft-form  space  which  lies  between  the  inner  surface  of  the 
orbiculus  ciUaris  and  the  corresponding  portion  of  the  anterior  border 
layer  of  the  vitreous.  I  (184)  have  called  this  space  the  orbicular  space, 
taking  this  name  from  Garnier  (72),  although  it  is  not  used  by  this  author 
in  exactly  the  same  sense  (PI.  I). 

Behind,  the  orbicular  space  has  no  sharp  border;  its  most  posterior 
portion  is  extensively  bridged  over  by  the  vitreous  fibrillae  and  very 
gradually  passes  over  into  the  body  of  the  vitreous  through  the  zonular 
cleft  (see  p.  153).  The  depth  of  the  orbicular  space  is  very  slight  at  the 
posterior  end,  not  greater  than  the  thickness  of  the  layer  of  zonula  fibers, 
i.e.,  some  o.oi  mm.  Here  the  space  is  in  reality  wholly  filled  out  by  the 
zonula  fibers.  In  the  anterior  part,  however,  the  border  layer  of  the 
vitreous  is  pressed  away  by  the  various  projections  of  the  ciliary  body, 
the  largest  zonula  fibers  are  displaced  to  the  inner  smooth  wall  of  the  space, 
and  outwardly  there  remains  a  relatively  free  space,  traversed  only  by  the 
fine,  straight,  and  backward-coursing  fibers.  The  space  is  only  present 
where  depressions  have  arisen  between  adjacent  projections  of  the  ciliary 
body.  In  such  places  the  depth  of  the  orbicular  space  may  amount  to 
o.  I  mm. 

In  front,  the  orbicular  space  goes  over  into  the  second  portion,  the 
system  of  ciliary  valleys  (PI.  VII,  2).  Since  the  anterior  border  layer 
of  the  vitreous  is  closely  apposed  to  the  posterior  halves  of  the  ciliary  pro- 
cesses, the  corresponding  portions  of  the  ciliary  bodies  are  closed  off  into 
short  canals  which  only  communicate  with  one  another  in  so  far  as  the 
vitreous  is  not  grown  to  the  ciliary  processes ;  it  only  lies  close  to  them. 

In  the  meridional  direction  these  canals  measure  some  o .  8  to  i .  o  mm, 
in  the  equatorial  direction  some  o. 3  at  the  posterior  end,  in  front  o .  2  mm, 
while  their  depth  (in  a  radial  direction)  increases  from  behind  forward  up 
to  something  like  o .  5  mm.  In  the  middle  of  the  corona  ciliaris  the  sur- 
face of  the  vitreous  bows  away  toward  the  lens,  and  the  anterior  halves  of 
the  ciliary  valleys  are  thereby  transformed  into  furrows,  which  in  their 
course  open  up  into  the  succeeding  division  of  the  posterior  chamber. 
The  arrangement  of  the  zonula  fibers  in  the  ciliary  valleys  has  been 
described  on  pp.  157-158.  Aside  from  these,  there  still  remains  abundant 
space  for  the  aqueous. 


CHA:\IBERS  of  eyeball  and  topography  of  THLS  region     175 

The  third  division  of  the  posterior  chamber  is  the  circumlental  space- 
By  this  usually  one  understands  only  the  space  between  the  crests  of  the 
cUiary  processes  and  the  border  of  the  lens,  as  a  matter  of  fact,  therefore, 
only  a  linear  dimension,  which  is,  however,  of  great  importance  to  physi- 
ology; yet  its  constant  presence  in  all  ages'  and  in  all  states  of  contraction 
of  the  ciliary  muscle  proves  it  only  acts  by  the  mediation  of  the  zonula 
and  in  any  case  possibly  of  the  vitreous  (Tscherning,  228). 

Considered  as  an  actual  space,  its  borders  are:  outward,  the  crests 
of  the  ciliary  processes;  inward,  the  equatorial  portions  of  the  lens; 
behind,  the  border  layer  of  the  vitreous.  In  front  it  really  has  no  border; 
for  the  sake  of  nomenclature  alone,  one  can  think  of  the  most  anterior 
zonula  iibers  as  forming  this  border.  Its  form  is  angular.  Frontally 
it  measures  scarcely  0.5  mm;  its  expanse  in  the  sagittal  direction  depends 
upon  the  form  of  the  lens  border.  It  is  somewhat  narrower  on  the  nasal 
side  than  upon  the  temporal. 

The  fourth  division  is  the  prezonular  space  (Czermak,  34),  or  the 
posterior  chamber  in  the  stricter  sense.  This  space  lies  between  the 
posterior  surface  of  the  iris,  on  the  one  hand,  the  anterior  surface  of 
the  lens,  the  most  anterior  zonula  fibers  and  the  anterior  declivities  of  the 
ciliary  processes,  on  the  other  hand.  It  opens  into  the  circumlental  space 
behind  through  the  spaces  between  the  zonula  fibers,  at  the  periphery 
into  the  ciliary  valleys,  axialward  through  the  pupil  into  the  anterior 
chamber. 

The  prezonular  space  has  its  greatest  depth  over  the  crests  of  the  ciliary 
processes  (0.4  to  0.6  mm) ,  from  thereon  very  gradually  narrows  toward 
the  pupil  and  disappears  a  short  distance  before  the  pupil-border  of  the 
iris  is  reached.  Toward  the  periphery  it  narrows  rapidly  and  at  the  same 
time  forms  there  a  narrow  angle.  Since,  however,  the  ciliary  processes 
reach  a  little  over  onto  the  posterior  surface  of  the  iris,  the  periphery  of 
the  prezonular  space  has  a  wavy  or  cogged  form. 

The  prezonular  space  is  the  only  division  of  the  posterior  chamber 
which  has  no  zonula  fibers  and  contains  only  aqueous. 

The  older  anatomists  conceived  of  the  zonula  as  a  folded  membrane  running  from 
the  ciliarj'  processes  to  the  lens.  Their  posterior  chamber,  therefore,  did  not  reach 
farther  than  the  zonula  and  corresponded  to  our  prezonular  space.  The  space  between 
the  zonula  and  the  \'itreous  was  called  Petit's  canal.  The  Petit's  canal  about  coincides 
with  the  circumlental  space,  yet  it  appears  that  after  certain  methods  of  preparation, 
e.g.,  after  insufflation  of  air,  sections  of  the  posterior  chamber  lying  still  farther  back- 
ward become  dravv-n  into  connection  with  it. 

One  can  inflate  the  so-called  Petit's  canal  with  air  because  of  the  surface  tension 
of  the  surrounding  fluid;  air  is  imprisoned,  just  as  one  can  occasionally  observe  to  his 


'  Exceptionally  it  disappears  in  senility  (chap.  xix). 


176  ANATOMY  AND  HISTOLOGY  OF  THE  HUMAN  EYEBALL 

discomfort  in  celloidin  imbedding,  between  or  beneath  the  Z(.)nula  libers  in  such  a 
way  that  one  cannot  bring  it  out  by  mechanical  means.  Yet  as  soon  as  one  attempts 
to  inject  the  postulated  canal  with  a  colored  aqueous  solution,  this  at  once  goes  over 
into  the  prezonular  space. 

b)   The  Anterior  Chamber 

Of  much  simpler  form  than  the  posterior  chamber,  the  anterior 
chamber  is  bordered  in  front  by  the  cornea,  by  the  trabeculum  of  the 
iris  angle  at  the  periphery,  and  behind  by  the  anterior  surface  of  the  iris 
and  that  portion  of  the  anterior  surface  of  the  lens  which  at  the  time  is 
exposed  in  the  pupil. 

The  frontal  diameter  of  the  anterior  chamber  amounts  to  1 1 . 3  to 
12.4  mm,  and  is,  therefore,  about  equal  to  the  horizontal  diameter  or 
the  anterior  surface  of  the  cornea;  the  vertical  diameter  is  as  great  as 
the  horizontal.  The  greatest  depth  of  the  anterior  chamber  is  found 
in  the  middle  and  corresponds  to  the  pupil  (some  2.8  mm). 

That  which  one  designates  as  the  chamber  depth  in  the  dioptric  system  of  the  eye 
is  not,  however,  this  value,  but  the  distance  of  the  anterior  surface  of  the  cornea  from 
the  anterior  surface  of  the  lens,  because  one  can  ignore  the  refraction  of  the  posterior 
surface  of  the  cornea  for  optical  purposes.  The  optical  chamber  depth  amounts  to 
about  3.6  mm. 

The  individual  variations  of  the  chamber  depth  are  considerable.  Tscherning  (228) 
added  together  64  measurements  of  various  authors  (the  most  of  them  from 
MaklakofT) ;  the  chamber  depths,  measured  ophthalmometrically,  vary  between  2 . 2 
and  5.1  mm;  most  frequently  these  values  lie  between  3.4  and  4.1  mm. 

Toward  the  periphery  the  depth  of  the  anterior  chamber  gradually 
decreases;  yet  its  minimum  does  not  always  lie  at  the  border,  but  fre- 
quently somewhat  farther  axialward,  somewhere  between  the  border  zone 
of  the  iris  and  the  border  of  Descemet's  membrane.  Only  in  cases  in  which 
the  iris  thins  out  very  gradually  toward  the  ciliary  border,  is  the  periphery 
of  the  anterior  chamber  formed  by  an  angle  somewhat  rounded  off  at 
its  apex.  When,  however,  as  is  usually  the  case,  the  thickness  of  the  iris 
is  not  essentially  changed  in  the  region  of  the  ciliary  zone,  the  anterior 
surface  of  the  iris  descends  abruptly  toward  the  ciliary  body,  and  then  the 
outermost  zone  of  the  anterior  chamber  or  the  portion  adjacent  to  the 
trabeculum  (the  iris  angle,  or  the  chamber  bay)  is  wider  than  its  entrance 
from  the  anterior  chamber  proper.  The  shallower  the  anterior  chamber, 
in  and  of  itself,  the  more  noticeable  is  this  difference,  the  more  plainly  does 
the  chamber  bay  appear  bowed  backward. 

Czermak  (36)  called  attention  to  the  significance  of  this  configuration  for  the 
pathology  of  the  eye  and  in  this  way  established,  as  it  appears  to  me,  the  only  plausible 
explanation  of  the  origin  of  peripheral  synechia. 


CHAMBERS  OF  EYEBALL  AND  TOPOGRAPHY  OF  THIS  REGION     177 

The  trabeculum  of  the  iris  angle  does  not  form  a  continuous  wall; 
the  numerous  spaces  in  it  stand  in  free  communication  with  the  iris  angle 
itself,  so  that  the  aqueous  can  bathe  the  wall  of  the  Schlemm's  canal. 
This  system  of  spaces  is  often  called  Fontana's  spaces,  because  it  was  held 
to  be  an  analogue  of  the  canalis  fontanae. 

The  reasons  which  speak  against  the  use  of  this  name  have  been  brought  out  in 
extenso  by  Rochon-Duvigneaud  (1S2),  and  especially  by  H.  Virchow  (234).  Fontana's 
canal  is  bound  up  in  the  existence  of  a  genuine  "iigamcntum  pecthtatum,"  i.e.,  in  the 
presence  of  arrow-like  iris  processes.  These,  however,  do  not  occur  at  all  in  man; 
when  iris  processes  are  present,  they  course  in  the  plane  with  the  anterior  surface  of 
the  iris  to  the  trabeculum.  But  even  these  are  not  present  in  all  eyes,  and  when  they 
do  occur,  are  so  sparse  that  they  cannot  be  made  use  of  as  a  limitation  of  a  space. 
According  to  Rochon-Duvigneaud  the  whole  iris  angle  is  much  more  analogous  to 
Fontana's  canal. 

The  anterior  chamber  possesses  an  endothelial  lining  which  is  con- 
tinuous over  the  cornea  and  through  the  trabeculum,  incomplete,  however, 
over  the  iris  (spaces  at  the  crj-pts),  and  in  the  territory  of  the  pupil 
it  is  entirely  absent. 

c)  Content  of  the  Chambers 

Both  chambers  contain  aqueous,  i.e.,  a  completely  clear,  colorless, 
odorless,  watery  fluid  of  alkaline  reaction.  According  to  Leber  (138), 
the  specific  gravity  in  man  is  1.0034  to  1.0036,  the  content  in  solid 
substance  is  0.82  per  cent,  including  albuminous  bodies,  yet  this  is  so 
slight  a  mass  that  the  aqueous  gives  no  notable  coagulum  on  hardening. 
In  the  microscopical  preparation,  therefore,  the  chamber  appears  to  be 
filled  out  only  by  the  imbedding  mass  ("empty").  Any  other  content 
is,  therefore,  pathologic. 

The  index  of  refraction  of  the  aqueous  is  i  .33366  to  i  .33485,  accord- 
ing to  Freytag  (61),  and  is,  therefore,  so  very  little  different  from  the 
index  of  the  vitreous  that  for  the  purpose  of  the  diopteric  system  one  is 
accustomed  to  think  of  a  uniform  medium  in  front  of  and  behind  the  lens. 

d)   Topography  of  the  Anterior  Segment 

The  waterj'  content  of  the  chambers  and  the  physical  processes  to  which  this  is 
subjected  in  the  dead  eye,  as  well  as  the  relative  slight  fi.xation  of  many  of  the  parts 
which  border  the  chambers,  brings  it  about  that  the  normal  relations  in  position  are 
easily  disturbed.  Therefore  we  should  not  look  upon  the  relations  found  in  the  com- 
pleted sections-preparation  as  corresponding  necessarily  to  those  present  in  life. 

The  depth  of  the  anterior  chamber  is  most  easily  extensively  changed.  Moreover, 
fluid  still  filters  into  Schlemm's  canal  after  death,  the  eye  becomes  soft,  and  the  anterior 
chamber  shallow.  The  diffusion  process  between  the  almost  pure  water  of  the  chamber 
and  the  relatively  concentrated  fixation  fluid  can  also  lessen  the  volume  of  the  anterior 


178  ANATOMY  AND  HISTOLOGY  OF  THE  HUMAN  EYEBALL 

chamber  in  the  eye  enucleated  durinp;  life,  wholly  aside  from  shrinkage  processes  in  the 
tissues  of  the  eye,  so  it  comes  about  that  we  have  before  us  ordinarily  an  abnormally 
shallow  anterior  chamber  in  the  completed  preparation. 

The  decrease  of  chamber  depth  expresses  itself  in  a  throwing  forward  of  the  lens, 
and  this,  again,  as  long  as  the  lens  is  plastic  enough,  is  combined  with  a  greater  vaulting 
of  its  anterior  surface.  The  iris  thereby  aj)i>roaches  the  cornea  and  its  conical  form  is 
increased. 

Still  more  striking  is  the  infllience  of  the  dislocation  of  the  lens  upon  the  form  of  the 
circumlental  space.  The  border  of  the  lens  is  shifted  forward  almost  as  much  as  the 
anterior  lens  pole;  the  ciliary  body,  on  the  other  hand,  remains  pretty  much  in  place. 
The  narrower  the  circumlental  space  is,  the  more  must  it  be  distorted,  the  more  must  the 
direction  of  the  zonula  fibers  which  pass  through  it  be  changed. 

A  second  process  which  disturbs  the  topography  is  the  shrinking  of  the  tissue. 
The  changes  which  the  whole  process  of  fixation  and  hardening  bring  about  in  the  lens 
have  already  been  emphasized  (p.  163).  The  shortening  of  the  equatorial  diameter  of 
the  lens  widens  the  circumlental  space  and,  indeed,  to  about  double  its  natural  size- 
The  zonula  fibers  become  tensely  spanned,  and  this  pull  must,  on  its  part,  be  again 
expressed  at  the  ciliary  body.  Microscopic  distortions  at  the  insertion  point  of  the  zonula 
fibers  are  the  result  of  hardening,  but  in  no  case  the  result  of  strained  accommodation. 

In  connection  with  the  shrinkage  of  the  vitreous  this  pull  also  leads  to  the  detach- 
ment of  the  ciliary  body  from  the  sclera.  Since,  however,  the  ciliary  body  is  firmly 
fastened  in  front  so  that  the  detachment  effects  a  rotation  of  the  whole  ciliary  body 
about  its  punctum  fixum,  the  anterior  surface  must  approach  more  the  frontal,  the 
inner  surface  more  the  sagittal  direction.  The  chorioidea  naturally  follows  this  detach- 
ment, at  least  the  anterior  portions  and  the  whole  perichorioidal  space  appears  unnatu- 
rally wide. 

But  even  when  one  succeeds  in  preventing  all  this,  the  shrinking  of  the  imbedding 
mass  can  still  change  the  topographic  relations.  The  only  way  to  get  around  these 
artefacts  is  to  make  use  of  fresh  material,  which  one  freezes  before  cutting.  But  only 
the  grosser  anatomic  relations  can  be  studied  and  measured  in  this  way;  finer  details 
must  be  added  afterward  to  the  survey  picture  obtained.  Some  dimensions  can  be 
established  during  life,  e.g.,  the  ophthalmometer  is  a  very  desirable  and  useful  means 
of  testing  the  correctness  of  topographic  relations  and  especially  for  adjudicating  them. 

How  great  the  difference  in  the  position  of  the  parts  can  be  is  seen  best  by  a  com- 
parison of  Text  Fig.  I  (p.  3)  and  PL  I;  Text  Fig.  i  shows  the  normal  topographic 
relations  from  the  above-presented  points  of  view,  corrected  as  necessary;  PI.  I,  on  the 
other  hand,  shows  the  relations  of  a  cadaver-eye,  which,  in  general,  has  been  very  well 
conserved. 

Topographic  relations  can  be  much  more  easily  reproduced  by  draw- 
ing than  by  description.  It  is,  therefore,  only  necessary  really  to  refer 
to  Text  Fig.  I ;  still  I  would  emphasize  here  certain  particularities,  partly 
because  of  their  clinical,  partly  because  of  their  physiologic  importance. 

Since  the  frontal  diameter  of  the  anterior  chamber  is  about  equal  to 
that  of  the  horizontal  diameter  of  the  cornea,  the  root  of  the  iris  lies  pretty 
nearly  behind  the  outer  border  of  the  cornea  in  the  horizontal  meridian, 
and  the  removal  of  it  from  this  (in  the  sagittal  direction)  is  i .  6  to  i .  S  mm. 


CHAMBERS  OF  EYEBALL  AXD  TOrOGRAPHY  OF  THIS  REGION     170 

Above  and  below,  however,  the  iris  root  lies  considerably  farther  equatorial. 
When,  therefore,  one  plunges  a  knife  into  the  cornea  i  mm  from  its  border, 
and  presses  the  blade  forward  parallel  to  the  base  of  the  cornea,  the  inner 
wound  falls  far  inside  (axial  to)  the  border  of  Descemet's  membrane. 

The  height  of  the  cornea  is  2 . 6  mm.  The  plane  of  the  iris  root, 
therefore,  lies  at  least  4 . 2  mm  behind  the  vertex  of  the  cornea.  Since  the 
distance  of  the  lens  from  the  vertex  of  the  cornea  is  3  . 6  mm,  the  anterior 
lens  pole  thus  projects  at  least  o .  6  mm  in  front  of  the  plane  of  the  iris  root. 
The  iris,  therefore,  forms  a  fiat  cone,  especially  in  a  narrow  pupil. 

It  is  a  necessary  result  of  this  conical  form  of  the  iris  that  the  sphincter 
is  pressed  against  the  lens  with  a  certain  component.  This  component 
is,  however,  only  a  small  fraction  of  the  whole  force  of  the  sphincter,  for 
it  is  proportional  to  the  sinus  of  that  angle  which  the  meridian  of  the 
iris  forms  with  the  frontal  plane.  The  steeper  the  iris  is  inclined  (the 
narrower  the  anterior  chamber)  the  greater  is  this  component. 

Now,  according  to  the  observation  of  Ulbrich  (229),  it  appears  that 
this  component  is  great  enough  to  prevent  for  a  time  the  passage  of 
fluid  out  of  the  posterior  into  the  anterior  chamber.  Only  in  this  very 
limited  sense  can  one  speak  of  a  physiologic  seclusion  of  the  pupil,  never, 
however,  in  the  strict  sense  in  which  Hamburger  (86)  uses  this  phrase. 

The  lateral  portions  of  the  anterior  lens  surface  (lying  behind  the  iris), 
the  most  anterior  zonula  fibers  and  the  inner  surface  of  the  ciliary  body 
(considered  as  a  whole  with  the  exception  of  the  processes)  lie  almost 
exactly  in  one  plane,  and  together  form  likewise  almost  a  single  conical 
mantle.  This  conical  mantle  under  certain  circumstances,  especially  in  a 
shallow  anterior  chamber,  may  take  on  a  certain  concavity  from  the  sims 
of  the  ciliary  body.  Attention  was  first  directed  to  this  concavity  by 
Schoen  (192),  as  an  important  support  for  his  theory  of  accommodation, 
yet  he  has  probably  emphasized  it  unduly,  and  the  preparations  depicted  by 
him  as  conclusive  are  not  free  from  disturbances  of  topographic  relations. 

The  apices  of  the  ciliary  processes,  i.e.,  the  points  farthest  axialward, 
lie  distinctly  in  front  of  the  lens  equator  and  the  wall  about  the  petellar 
fossa  of  the  vitreous  lies  on  the  posterior  halves  of  the  corona  ciliaris. 

It  is  only  because  of  the  magnification  produced  by  the  cornea  that 
one  cannot  see  the  iris  throughout  its  entire  extent  during  life.  For  the 
same  reason  in  a  given  case,  e.g.,  in  iridodialysis,  one  can  see  only  the 
apices  of  the  ciliary  processes,  and  when  viewed  from  in  front  the  border 
of  the  lens  almost  coincides  with  the  border  of  the  cornea,  although  the 
frontal  diameter  of  the  lens  is  2  to  3  mm  smaller  than  the  horizontal  diame- 
ter of  the  cornea.  The  magnification  occasioned  by  the  cornea  increases, 
indeed,  with  the  depth;    thus  the  iris  appears  magnified  one-ninth,  the 


i8o  ANATOMY  AND  HISTOLOGY  OF  THE  HUMAN  EYEBALL 

lens  on  ihe  other  hand,  one-fifth,  if  one  conceives  of  the  plane  of  the  lens 
e(|uator  as  5  mm  removed  from  the  vertex  of  the  cornea.  A  structure 
lying  in  the  focal  point  of  the  cornea,  e.g.,  a  posterior  polar  cataract, 
must  appear  magnified  one-third. 

The  refraction  of  the  cornea  furthermore  emphasizes  the  iris,  therefore 
makes  the  anterior  chamber  appear  shallower  than  it  actually  is.  This 
principle  holds  true  for  everything  which  lies  between  the  anterior  sur- 
face of  the  cornea  and  the  focal  point  of  it  (more  accurately  the  nodal 
point  of  the  whole  eye). 

Of  course,  the  figures  given  apply  only  to  an  average  (schematic  eye). 

CHAPTER  XV.      THE  VESSELS  AND  NERVES  OF  THE  EYEBALL 

So  far  as  these  are  constituent  parts  of  the  tissue,  they  have  already 
been  mentioned  in  connection  with  it.  Only  a  few  details  concerning  the 
supply  of  the  eyeball  with  blood-vessels  and  nerves  are  added  here  and 
the  relations  of  the  individual  circulatory  and  innervation  districts  dis- 
cussed. 

a)  Blood-Vcssds  of  the  Eyeball 

All  of  the  arteries  of  the  eyeball  are  branches  of  the  arteria  ophthalmica 
in  the  last  instance.  These  course  in  part  directly  to  the  posterior  parts 
of  the  eyeball  and  optic  nerve,  in  part  to  the  anterior  segment  by  means  of 
a  roundabout  way  along  the  eye  muscles.  The  veins  empty  their  blood 
into  the  orbital  veins,  and  by  means  of  this  partly  direct  into  the  sinus 
cavernosus,  although  anastomoses  of  the  orbital  veins  with  those  of  the 
face  also  occur. 

But  in  the  eyeball  itself,  there  can  be  differentiated  two  vessel  dis- 
tricts, according  to  Leber  (138),  whose  classical  presentation  I  follow  in 
the  main — the  retinal  or  inner,  and  the  ciliary  or  outer  vessel  system. 

I.      THE   RETINAL   SYSTEM 

In  the  adult  eye  this  is  represented  by  the  arteria  and  vena  centralis 
retinae.  Its  territory  is,  especially,  the  retina,  a  minor  portion  of  the 
optic  nerve  and  its  sheaths. 

The  arteria  centralis  retinae  enters  the  optic  nerve  7  to  1 2  mm  behind 
and  below  the  bulb.  It  first  supplies  the  neighboring  portions  of  the 
sheaths,  then  its  immediate  neighborhood  in  the  optic  nerve,  in  the  axis 
of  which  it  courses  farther  on  to  the  bulb.  Its  part  in  the  supply  of  the 
optic  nerve  is,  therefore,  a  minor  one;  only  at  the  lamina  crihrosa  does  it 
give  off  a  larger  number  of  fine  branches.     Here  and  in  the  intrachorioidal 


THE  VESSELS  AND  NERVES  OF  THE  EYEBALL  iSi 

section  of  the  optic  nerve  the  last  capillary  anastomoses  between  the 
retinal  and  the  ciliary  vessel  systems  are  found,  but  from  the  inner  end 
of  the  optic  nerve  canal  on,  the  retinal  is  completely  separated  from  the 
ciliary  system,  as  a  rule.  This  portion  of  the  arteria  centralis  retinae 
is  an  end  artery  in  the  sense  of  Cohnheim,  and,  furthermore,  all  of  the 
blood  carried  by  the  arteria  centralis  retinae  is  carried  away  by  the  vena 
centralis  retinae. 

The  arteria  centralis  retinae  divides  into  an  upper  and  lower  main 
branch  {arteriae  papillaris  superior  et  inferiores)  on  the  inner  surface  of  the 
papilla  and  these,  again,  into  a  nasal  and  temporal  branch  {arteriae 
nasales  super,  et  inf.,  arteriae  temporales  super,  et  inf.)  (PI.  VII,  i).  Yet 
even  in  respect  to  the  branches  of  the  second  order  there  rules  a  signifi- 
cantly lessened  regularity.  Under  further  gable-like  divisions  the  arteries 
broaden  out  into  the  retina  mostly  in  a  radial  direction;  the  temporal 
branches  alone  course  in  wide  bows  above  and  below  the  fovea  and, 
converging,  send  fine  branches  to  the  fovea.  Finally,  as  a  rule,  a  few 
fine  branches  go  directly  over  the  temporal  border  of  the  papUla  to  the 
fovea. 

Concerning  the  capillary  system  the  reader  is  referred  to  p.  82.  The 
last  extensions  in  the  neighborhood  of  the  ora  serrata  bow  about  in  loops 
into  the  veins.  These  communications  are,  indeed,  somewhat  larger 
than  the  capUlaries,  but  they  only  go  back  into  the  same  vessel  system, 
and  cannot,  therefore,  functionate  as  collateral  paths. 

The  distribution  of  the  veins  fully  corresponds  for  the  most  part  to 
that  of  the  arteries;  the  vena  centralis  retinae  accompanies  the  artery  of 
the  same  name  and,  along  with  it,  is  united  to  the  central  connective- 
tissue  strand  (pp.  91,  106)  by  an  extension  of  the  pial  sheath;  it  usually 
empties  directly  into  the  sinus  cavernosus. 

2.      THE    CILIARY   SYSTEM 

This  supplies  the  rest  of  the  coats  of  the  eyebaU,  the  neighboring  por- 
tions of  the  optic  nerve  and  its  sheaths,  as  well  as  the  conjunctiva.  The 
arterial  radical  of  this  system  divides  into  the  posterior  and  anterior 
ciliary  arteries;  the  venous  radical  is  made  up  of  the  vortex  veins  and  the 
anterior  ciliary  veins.  Arteries  and  veins  do  not  correspond  to  each 
other  in  this  system,  either  their  course  or  yet  in  circulatory  areas,  so  that 
the  arteries  come  out  more  strongly  in  one  place,  the  veins  in  another 
place,  especially  in  the  uveal  tract. 

The  number  of  the  posterior  ciliary  arteries  {art.  ciliares  posteriores) 
amounts  to  about  twenty.  They  surround  the  optic  nerve  and  enter  the 
ejeball  in  its  neighborhood  and  in  the  region  of  its  posterior  pole.     At 


i82  ANATOMY  AND  HISTOLOGY  OF  THE  HUMAN  EYEBALL 

first  they  are  distributed  to  the  episcleral  vessel  net  (as  far  as  the  in- 
sertions of  the  recti  muscles),  then  they  pass  into  the  sclera  (cmissaria; 
cf.  p.  i8). 

Most  of  them  pass  directly  out  of  the  sclera  into  the  chorioidea,  and 
become  known  as  the  short  posterior  ciliary  arteries  {art.  cil.  post,  breves). 
The  course  of  these  vessels  shows  great  variation;  some  press  into  the 
dural  sheath  directly  at  the  root  and  turn  toward  the  optic  nerve,  others 
enter  at  a  greater  distance;  sometimes,  too,  a  short  posterior  ciliary  artery 
branches  off  from  a  long  one  or,  united  with  it,  enters  an  emissarium. 

The  short  posterior  ciliary  arteries  supply  the  back  half  of  the  uveal 
tract  and  the  optic  nerve;  those  entering  in  the  neighborhood  of  the  dural 
sheath  form  a  circle  of  anastomoses  about  the  optic  nerve  by  means  of  a 
few  branches — the  cir cuius  arteriosus  nervi  optici  (p.  25).  The  neighbor- 
ing portions  of  the  pial  sheath,  especially  the  very  rich  vessel  net  of  the 
lamina  cribrosa,  are  supplied  by  this. 

Elschnig  (52)  has  called  attention  to  a  special  variation  in  the  terri- 
tory of  the  posterior  ciliary  arteries.  A  relatively  large  artery  enters 
the  dural  sheath  behind  the  bulb  some  3  mm  from  the  end  of  the  inter- 
vaginal  space  and,  splitting  the  dural  sheath  into  two  leaves,  courses  in 
this  to  the  sclera,  then  goes  along  the  insertion  of  the  dural  sheath  over 
into  a  circular  course,  and  extends  about  halfway  around  the  circumfer- 
ence of  the  optic  nerve,  dividing  up  into  finer  branches. 

According  to  Elschnig,  the  circuliis  arteriosus  nervi  optici  is  absent  in 
such  eyes;  according  to  my  observations  (185),  however,  it  is  not 
always,  but  the  branches  of  the  abnormal  artery  enter  the  circulus  or  go 
directly  into  the  chorioidea.  At  times,  too,  there  is  a  larger  recurrent 
branch  for  the  pial  sheath  and  the  medullary  section  of  the  optic  nerve. 
This  variation  occurs  in  about  half  the  eyes;  it  very  easily  escapes 
observation,  however,  because  the  vessel  does  not  lie  in  the  horizontal 
meridian  of  the  papilla. 

As  already  noted,  only  a  capillary  connection  exists  between  the  retinal 
and  ciliary  systems,  and  this  does  not  reach  beyond  the  level  of  the  chorio- 
capillaris.     This  rule,  however,  is  subject  to  many  exceptions. 

Anastomoses  of  larger  caliber,  visible  ophthalmoscopically,  are  rare 
and  occur  most  frequently  between  the  veins.  A  very  large,  band-like, 
flattened  vein  sometimes  branches  off  from  the  central  vein  or  one  of  its 
main  branches,  courses  across  through  the  tissue  of  the  papilla,  and  dis- 
appears beneath  the  margin  of  the  papilla  (optico-ciliary  vein,  Elschnig, 
49).     Oeller  (167)  depicts  an  analogous  artery. 

The  anatomic  proof  of  such  an  anastomosis  was  first  brought  by  Kuhnt 
(131);    however,  the  vessel  observed  by  him  could  not  have  been  visible 


THE  VESSELS  AND  NERVES  OF  THE  EYEBALL  183 

ophthalmoscopically  on  account  of  its  position  behind  the  lamina  cribrosa. 
Elschnig  (51)  then  demonstrated  a  tj'pical  optico-cUiary  vein  in  an  eye 
affected  with  a  neuritic  optic-nerve  atrophy.  It  went  off  from  the  central 
vein  immediately  in  front  of  the  lamina  cribrosa  and  emptied  into  the 
vessel  system  of  the  chorioidea.  In  choked  disk  and  other  similar  con- 
ditions such  venous  unions  have  been  shown  to  exist  heretofore,  yet  it 
is  questionable  whether  these  were  not  pathologic  distensions  of  originally 
capillary  unions. 

Much  more  frequently  it  happens  that  a  smaller  or  larger  territory 
of  the  retina  is  not  supplied  by  the  central  artery  but  by  branches  of  the 
cUiary  vessel  system,  or  that  it  does  not  empty  its  blood  into  the  central 
vein.     Such  abnormal  vessels  are  called  cilio-rctiiial. 

According  to  Elschnig  (50),  the  cilio-retinal  arteries  are  throughout 
derivatives  of  the  circulus  arteriosus  nervi  optici,  which  either  go  directly 
from  this  through  the  sclera  and  the  border  tissue  in  an  oblique  direction 
into  the  non-meduUated  section  of  the  optic  nerve,  or  take  the  roundabout 
course  through  the  chorioidea  and,  therefore,  appear  as  branches  of  the 
chorioidal  arteries.  In  both  cases  they  attain  the  intrachorioidal  section 
of  the  optic  nerve  and  bend  about  the  border  of  the  chorioidal  foramen  into 
the  retina,  where  they  are  distributed  like  typical  retinal  vessels.  This 
bending  is  visible  ophthalmoscopically  as  a  hooked  curve,  the  character- 
istic index  of  the  cilio-retinal  vessel. 

Cilio-retinal  (or,  as  Elschnig  calls  them,  retino-ciliary)  veins  have 
been  anatomically  demonstrated  in  only  a  single  instance  (Kuhnt,  131); 
in  this  case  the  vein  entered  the  sclera. 

In  the  older  ophthalmoscopic  observations,  especially  those  of  Nettleship  (165), 
there  is  much  discussion  about  cilio-retinal  veins.  It  has,  however,  been  emphasized 
by  Elschnig  (50)  that  cilio-retinal  veins  are  very  much  more  rare  than  cilio-retinal 
arteries  and,  independent  of  Elschnig,  I  have  come  to  the  same  conclusion.  The  well- 
estabhshed  retino-ciliary  veins  are  often  associated  with  other  similar  anomalies,  e.g., 
with  optico-ciliary  vessels  (Elschnig,  50)  or  with  abnormal  vortex  veins  (Czermak,  35). 

The  cilio-retinal  vessels  are  relatively  frequent;  Lang  and  Barrett  (133)  found  them 
in  16.7  per  cent  of  eyes,  I,  in  16.4  per  cent.  Elschnig  (50)  estimates  their  frequency 
at  only  7  per  cent,  but  possibly  has  in  mind  only  the  larger  vessels. 

Most  frequent  are  the  small  macular  vessels  (11  per  cent);  these  appear  at  the 
temporal  border  of  the  papilla,  and  go  directly  to  the  fovea.  Vessels  of  a  caliber  such 
that  the  direction  of  their  current  can  be  determined  with  certainty  ophthalmoscopically, 
are  found  in  some  6  per  cent  and  are  almost  exclusively  arteries.  They  often  represent 
those  branches  of  the  second  or  third  order  which  circle  about  the  fovea  in  a  bow.  Still 
larger  arteries  are  rare;  they  may  have  the  dignity  of  an  arteria  papillaris  and  then  their 
place  of  origin  is  displaced  just  as  much  farther  abo\'e  or  below  as  the  vessel  is  large. 
Upon  one  occasion,  I  observed  the  complete  absence  of  the  central  artery  and  the  sub- 
stitution of  two  cilio-retinal  arteries  for  this  vessel,  as  has  also  Bloch  (26). 


1 84  ANATOMY  AND  HISTOLOGY  OF  THE  HUMAN  EYEBALL 

The  great  majority  of  the  cilio-retinal  arteries  occur  in  the  temporal  half  of  the 
circumference  of  the  optic  nerve  and  supply  part  of  the  temporal  half  of  the  retina. 
Nasal  cilio-retinal  arteries  are  very  rare  and  often  associated  with  anomalies  of  the 
papilla. 

The  long  posterior  ciliary  arteries  {art.  cil.  poster,  longac)  are  charac- 
terized by  larger  caliber  and  course  in  the  horizontal  meridian;  there  are, 
therefore,  only  two  such  arteries,  one  on  the  nasal,  one  on  the  temporal 
side.  The  long  posterior  ciliary  arteries  pass  through  their  emissaria 
(p.  1 8)  and  the  perichorioidal  space  (p.  51)  without  giving  off  branches, 
press  into  the  ciliary  muscle  at  its  posterior  border,  and  divide  therein. 
These  branches  reach  to  the  anterior  surface  of  the  ciliary  body  and  there 
bend  about  along  the  root  of  the  iris,  but  go  over  into  the  circular  direction 
above  and  below,  always  coursing  in  the  ciliary  body.  By  union  of  these 
branches,  as  well  as  through  anastomoses  which  bridge  across  the  divi- 
sions, an  arterial  circle  is  constructed — the  circulus  iridis  major. 

The  anterior  ciliary  arteries  (art.  ciliares  anteriores)  come  from  the 
straight  eye  muscles,  and  accompany  each  tendon  in  pairs;  the  m.  rectus 
lateralis  is  usually  accompanied  by  only  one  artery.  The  little  trunks 
pass  over  the  insertion  of  the  tendon  in  the  episcleral  tissue  with  great 
tortuosity  to  within  a  distance  of  3  to  4  mm  of  the  cornea;  then  they 
divide  into  superficial  and  one  large  perforating  branch.  The  former 
supply  the  episcleral  vessel  net,  the  border  loop  net  of  the  cornea,  and 
the  bordering  zone  of  the  scleral  conjunctiva. 

The  perforating  branches  pass  through  the  sclera  steeply,  often  at 
almost  a  right  angle  (emissaria;  cf.  p.  18),  and  then  at  once  enter  the 
ciliary  muscle.  It  anastomoses  there,  partly  with  the  long  posterior 
ciliary  arteries  or  its  branches,  partly  with  the  circulus  iridis  major. 

The  system  formed  by  the  long  posterior  and  the  perforating  branches 
of  the  anterior  ciliary  arteries  supplies  the  anterior  half  of  the  uveal  tract, 
the  ciliary  muscle  first,  then  the  orbiculus  ciliaris,  and  the  anterior  portions 
of  the  chorioidea  by  means  of  backward-coursing  branches  (art.  recur- 
reiites),  while  the  cUiary  processes  (p.  115)  and  the  iris  (p.  133)  are  supplied 
by  the  circulus  iridis  major. 

The  vortex  veins  {venae  vorticosae)  are  the  most  important  of  the  veins 
of  the  ciliary  system  (cf.  pp.  9,  18,  52).  They  carry  away  almost  all  the 
blood  from  the  uveal  tract — the  blood  of  the  chorioidea,  of  the  ciUary 
processes,  and  of  the  iris — and,  moreover,  for  the  greater  part,  that  of  the 
ciliary  muscle.  The  blood  takes  another  course  only  in  the  anterior  parts 
of  the  ciliary  muscle,  i.e.,  through  small  veins,  the  anterior  ciliary  veins 
{venae  ciliares  anteriores),  which  go  from  the  ciliary  body  o\'er  into  the 
sclera  just  behind  the  scleral  roll,  take  up  there  the  drainage  of  the 


THE  VESSELS  AXD  NERVES  OF  THE  EYEBALL  185 

Schlemm's  canal,  and  finally  attain  the  episcleral  tissue  in  the  neighbor- 
hood of  the  border  of  the  cornea.  Here  they  drain  the  marginal  loop  net 
and  the  neighboring  conjunctival  zone,  and  with  these  and  the  episcleral 
vein  form  a  richly  divided  net,  which,  like  the  vortex  veins,  empties  into 
the  orbital  veins. 

The  posterior  portions  of  the  episcleral  tissue  have  their  own  small 
veins  in  the  neighborhood  of  the  optic  nerve.  The  neighborhood  of  the 
optic  nerve  is  very  poor  in  veins  as  a  result  of  this,  and  larger  veins  do 
not,  in  general,  normally  occur  here.  The  posterior  ciliary  arteries,  which 
course  in  the  dural  sheath,  are  only  occasionally  accompanied  by  large 
veins  (Elschnig,  52). 

Abnormal  vortices  have  occasionally  been  observed  in  myopic  eyes  at 
the  border  of  the  chorioidal  foramen,  more  rarely  in  eyes  of  normal  form. 
Whether  or  not  they  are  actually  more  frequent  or  can  only  be  more  easily 
seen  with  the  ophthalmoscope  in  such  eyes  (on  account  of  the  atrophy 
of  the  pigment  epithelium),  remains  still  to  be  decided.  Most  of  the  cases 
have  been  observed  ophthalmoscopically  only.  Axenfeld  and  Yamaschita 
(12)  alone  have  made  a  short  report  of  such  an  anatomic  finding. 


There  is  not  much  to  be  said  concerning  the  vessels  of  the  eye,  his- 
tologically; their  structure  corresponds  to  the  type.  The  muscularis 
of  the  arteries  is  more  weakly  developed  inside  the  eye  than  in  the  orbit; 
this  probably  depends  upon  the  fact  that  the  walls  of  the  intraocular 
vessel  have  only  to  bear  the  difference  between  the  blood  pressure  and 
the  intraocular  pressure.  One  often  comes  upon  the  statement  that  the 
muscularis  is  absent  in  the  arteries  (in  the  retina  and  iris).  This  is  not 
correct;  smooth  muscle-fibers  of  the  wall  can  be  followed  even  into  the 
finer  branches. 

The  veins  are  everpvhere  pro\'ided  mth  perivascular  sheaths;  in 
general,  their  wall  consists  of  connective  tissue;  muscle-fibers  appear  only 
in  the  vortex  veins  in  the  neighborhood  of  the  outer  surface  of  the  sclera. 
According  to  my  observations,  these  are  directed  crosswise  or  obHquely. 

b)  Lymph  Passages 

True  lymph  vessels  occur  only  in  the  scleral  conjunctiva,  not,  however, 
in  the  eyeball  itself,  nor  in  the  orbit.  On  the  other  hand,  larger  spaces 
are  present,  which,  among  other  purposes,  serve  to  a  greater  or  lesser 
extent  for  the  movement  of  lymph.  In  this  category  belong  the  inter- 
vaginal  space  of  the  optic  nerve,  the  perichorioidal  space,  and  especially, 
the  posterior  and  anterior  chambers. 


i86  ANATOMY  AND  HISTOLOGY  OF  THE  HUMAN  EYEBALL 

In  times  past  one  laid  very  great  weight  upon  the  results  of  injections, 
and  all  the  pictures  which  arose  with  a  certain  regularity  were  attributed 
to  preformed  lymph  channels.  But  how  very  little  such  experiments 
prove  is  shown  by  the  example  of  the  cornea,  in  which  the  idea  of  pre- 
formed channels  for  the  lymph  stream  has  been  entirely  given  up.  Even 
a  splitting  of  tissue  can  occur  systematically,  especially  where  various 
component  parts  border  upon  one  another. 

The  investigation  of  the  lymph  circulation  and  its  paths  now  demand 
wholly  other  methods,  which  do  not  fall  any  more  within  the  province 
of  anatomy  and  histology;  I  must,  therefore,  refer  the  reader  to  the  proper 
treatises,  especially  that  of  Leber  (138),  for  an  extended  presentation. 

c)  Nerves  of  the  Eye  {N.  ciliares) 

Heretofore,  posterior  and  anterior  ciliary  nerves  were  distinguished. 
Axenfeld  (10)  has,  however,  shown  that  these  so-called  anterior  ciliary 
nerves  are  in  most  cases  really  posterior  ciliary  nerves  which  only  vary 
from  the  typical  in  their  course.'  The  posterior  ciliary  nerves  are,  there- 
fore, in  short,  the  ciliary  nerves.  They  spring  partly  from  the  ganglion 
ciliare  {11.  cil.  breves),  partly  from  the  n.  naso-ciliaris  {n.  cil.  longi);  the 
latter  carry  sensory  nerves  to  the  eyeball,  the  former,  a  mixture  of  three 
kinds  of  fibers — sensory,  motor  and  sympathetic  fibers.  The  long  ciliary 
nerves  also  lie  in  the  neighborhood  of  ganglion  ciliare  and  unite  with  the 
short  nerves  in  the  neighborhood  of  the  optic  nerve  inside  the  sclera. 

As  a  result  of  this,  it  is  possible  to  render  the  eyeball  completely  insensible  by 
cutting  the  ciliary  nerves  at  their  entrance  into  the  eyeball  {neurotomia  optico-ciliaris), 
also  by  injection  of  cocaine  in  the  region  of  the  ganglion  ciliare  (method  of  regional 
anaesthesia  of  Elschnig;  see  Loewenstein,  145). 

The  number  and,  corresponding  to  it,  the  size  of  the  nerve  trunks 
before  their  entrance  into  the  sclera  seems  to  be  subject  to  a  wide  variation. 
One  can  usually  make  out  about  ten  of  the  larger  nerves. 

In  the  orbit  the  ciliary  nerves  show,  outermost,  a  connective-tissue 
coat  (neurilemma),  after  which  there  follows  a  second  coat  of  fiat  proto- 
plasmic cells — several  layers  in  larger  nerves — and  mixed  fine  collagenous 
fibers.  According  to  Gutmann  (82),  the  nerve-fibers  are  almost  exclu- 
sively medullated;  according  to  Hahn  (84),  they  are  exclusively  so; 
their  caliber  varies  from  20  mu  down  to  the  greatest  fineness.  Most  fre- 
quently I  find  fibers  of  7  to  1 2  mu  thickness,  between  which  the  fine  fibers 
lie  in  small  groups.  All  have  nucleated  (Schwann)  sheaths.  The  spaces 
between  the  nerves  are  filled  out  by  fine  collagenous  fibers. 


'  Ki.  the  most  genuine  anterior  ciliary  nerves  can  occasionally  be  derived  from  the  n.  naso-ciliaris. 


THE  VESSELS  AND  NERVES  OF  THE  EYEBALL  1S7 

Furthermore,  the  ciliary  nerves  frequently  contain  ganglion  cells  in 
the  neighborhood  of  the  sclera,  indeed,  even  in  the  latter  itself,  and  since 
these  ganglion  cells  often  form  little  groups  one  may  speak  of  accessory 
episcleral  ciliary  ganglia  (Axenfeld,  11). 

In  the  neighborhood  of  the  optic  nerve  one  finds  many  small  nerve 
branches  in  the  sclera  (often  consisting  of  only  a  few  fibers) ;  the  course 
of  these  branches  is  very  irregular.  The  larger  ciliary  nerves,  however, 
course  in  very  oblique  emissaria  (p.  18).  One  ciliary  nerve  regularly 
accompanies  each  of  the  long  posterior  ciliary  arteries.  While  still  within 
the  emissarium  this  gives  off  a  weaker  branch,  which  crosses  the  ciliary 
artery,  so  that  the  artery  at  its  entrance  into  the  perichorioidal  space  is 
accompanied  by  two  nerves  (p.  51). 

With  the  entrance  into  the  emissarium  the  ciliary  nerve  loses  its 
connective-tissue  coat;  the  cell-coat  is  also  reduced  in  its  further  course 
through  the  perichorioidal  space  to  a  simple  layer  of  flat  cells.  In  the 
place  of  them  suprachorioidal  lamellae  appear  as  coats  of  the  nerves  in 
this  space;    the  nerve  itself  has  an  elliptical  cross-section. 

The  ciliary  nerves  traverse  the  perichorioidal  space  in  a  meridional 
direction  and  give  off  branches  to  the  sclera  and  to  the  chorioidea  while 
doing  so  (p.  51).  The  scleral  branches,  however,  only  partly  supply 
this  tunic  (p.  25);  in  addition  they  bore  through  the  sclera  and  course 
to  the  cornea  through  the  episcleral  tissue  or  the  sclera  itself.  On  cursory 
examination,  these  branches  seem  to  be  anterior  ciliary  nerves.  The 
intrascleral  nerve  loops  (pp.  18-19),  which  are  occasionally  found,  can 
also  give  rise  to  the  same  confusion. 

The  main  mass  of  the  fibers  of  the  ciliary  nerves  enters  the  plexus 
ciliaris  in  the  ciliary  muscle  (p.  114).  This  supplies  the  ciliary  muscle 
itself  and  provides  the  nerves  for  the  ciliary  processes,  the  iris,  and  for 
the  deeper  layers  of  the  cornea.  These  latter  enter  the  sclera  behind  the 
scleral  roll  and  course  by  a  short  route  to  the  cornea.  The  superficial 
layers  of  the  cornea  are  supplied  by  the  perforating  branches  lying  farther 
backward.  Concerning  the  finer  subdivisions  of  the  nerve,  the  reader  is 
referred  to  the  portion  of  the  tissue  concerned. 


PART  II 

THE  PHYSIOLOGIC  CHANGES  OF  THE  EYEBALL  DURING  LIFE 
(Development  and  Senescence) 


That  which  the  reader  has  come  to  know  in  Part  I  is  the  structure  of 
the  eyeball  at  the  height  of  its  development — after  the  completion  of  the 
body  growth.  Before  this  period  in  life  there  lies  the  period  of  develop- 
ment, after  it  comes  the  period  of  senescence.  And  although  some 
details  have  already  been  reported  on  account  of  their  connection  with 
development  and  senescence,  there  yet  remains  a  great  deal  to  say, 
especially  concerning  the  development,  if  the  conception  of  the  eye  in  the 
different  periods  of  life  is  to  be  a  complete  one,  if  the  anatomic  structure 
and  the  significance  of  its  parts  is  to  be  rightly  understood. 


CHAPTER  XVI.     THE  EMBRYONAL  AND  FETAL  DEVELOPMENT 

So  rich  a  material  has  now  been  brought  together  in  the  normal  charts  of  Keibel 
(ii6),  by  the  thoroughgoing  researches  of  Seefelder,'  and  many  other  authors,  that  we 
may  now  consider  the  developmental  history  of  the  human  eye  to  have  been  worked 
out,  in  the  strict  sense  of  the  word.  I,  myself,  have  to  thank  the  friendly  consideration 
of  the  director  of  the  L  Academic  Institute  of  Human  Anatomy  of  Vienna,  Professor 
Jul.  Tandler  for  the  opportunity  of  using  the  rich  collection  of  the  institute  to  orient 
myself  at  least  concerning  the  most  important  phases  of  the  development  of  the  eye  by 
personal  observation. 

I  have  not  gone  into  the  details  of  histogenesis,  but  refer  to  the  cited  literature 
in  this  connection.  Such  a  task  demands  a  specialist  in  embryology,  and  that  I  am 
not.  For  the  same  reason  I  have  intentionally  avoided  still  undecided  matters.  I 
must  also  emphasize  that  the  following  should  be  looked  upon  only  as  a  sketch  of 
the  developmental  history  of  the  human  eye,  making  pretense  to  neither  originality 
nor  completeness. 

Since  the  estimation  of  the  age  of  very  young  embryos  is  extremely  difficult  in  the 
human,  it  is  now  preferable  to  give  only  the  greatest  length  (gr.  1.).  This  is  a  measure- 
ment of  the  body-axis  in  the  very  first  stages  only,  and  these  scarcely  come  into  con- 
sideration in  the  development  of  the  eye.  After  the  appearance  of  the  neck  bend,  the 
greatest  length  is  the  distance  from  the  neck  to  the  buttocks  (neck-buttocks  length), 
later  that  from  the  skull  to  the  buttocks  becomes  the  greatest  (skull-buttocks 
length,  SS).  In  the  later  stages  of  fetal  life  one  measures  this  length  or  the  skull-heel 
length,  i.e.,  the  length  of  the  fetus  with  outstretched  legs  (cf.  Michaelis,  154).  Unfor- 
tunately, an  accurate  notation  of  the  measurement  is  absent  in  many  statements  in 
which  simply  the  length  (1.)  appears  to  be  given. 

Even  in  the  earliest  phases  of  development  one  finds  considerable  individual  varia- 
tions in  respect  to  the  time  of  its  appearance.  In  such  cases  I  have  usually  taken  only 
the  lower  limit. 


■  The  large  treatise  by  Bach  and  Seefelder,  Atlas  of  the  Developmental  History  of  the  Human  Eye, 
Leipsic,  191 1,  first  began  to  appear  while  this  work  was  in  press,  and  could  not,  therefore,  be  made  use  of. 


192  ANATOMY  AND  HISTOLOGY  OF  THE  HUMAN  EYEBALL 

Even  before  the  complete  delimitation  of  the  medullary  canal  from  the 
rest  of  the  ectoderm,  therefore  before  the  time  when  there  is  still  an  open 
furrow  at  the  apical  end,  two  grooves,  the  optic  grooves,  appear  on  each 
side  of  the  median  line  on  the  floor  of  this  furrow.  That  which  presents 
itself  in  the  view  from  behind  as  a  groove  appears  in  the  view  from  the 
side  of  the  ventral  cavity  as  an  evagination,  and  in  cross-section  as  a  fold  of 
the  wall  of  the  medullary  canal ;  the  cap  of  this  fold  abuts  on  the  ectoderm 
(PI.  IX,  6;   copied  from  Keibel,  ii6,  No.  6,  p.  24;  Text  Fig.  6a). 

This  evagination  takes  on  a  lateral  direction  after  the  closure  of  the 
medullary  canal  and  becomes  transformed  into  a  vesicular  structure  (the 
primary  optic  vesicle)  in  a  gr.  1.  of  2 . 5  to  3  mm.  This  is  separated  from 
the  medullary  canal  on  the  dorsal  side  by  a  constriction,  the  pedicle  of  the 
optic  vesicle ;  at  the  same  time  the  medullary  canal  has  widened  into  the 
forebrain  at  its  anterior  (apical)  end.  On  the  ventral  side,  however,  the 
wall  of  the  optic  vesicle  goes  smoothly  over  into  that  of  the  forebrain. 
The  pedicle  of  the  optic  vesicle  is,  therefore,  very  short,  and  the  lumen 
of  the  optic  vesicle  stands  in  wide-open  communication  with  the  lumen  of 
the  forebrain. 

At  the  same  time,  several  layers  of  mesoderm  have  interposed  them- 
selves between  the  ectoderm  and  the  summit  of  the  optic  vesicle  (Fuchs, 
Textbook  of  Diseases  of  the  Eye,  12th  German  ed.,  Fig.  143;  4th  English 
ed..  Fig.  162).  The  optic  vesicle  is  very  poorly  separated  off  from  this, 
for  the  mesodermal  cells  enter  into  many  protoplasmic  connections  with 
the  cells  of  the  wall  of  the  optic  vesicle,  as  well  as  with  the  ectoderm 
(Seef elder,  203). 

This  mesodermal  layer  has,  however,  only  a  short  existence.  Cirin- 
cione  {^^)  first  demonstrated  that  this  layer  disappears  with  the  invagina- 
tion of  the  lens.  Even  at  a  gr.  1.  of  3 . 5  mm  (collection  of  the  I.  Anatomic 
Institute  of  Vienna),  the  mesodermal  cells  have  been  reduced  to  a  few 
scattered  remnants,  yet  the  unions  of  the  ectoderm  with  the  primary 
optic  vesicle  remain  constant  in  the  form  of  fine  protoplasmic  threads 
(embryonal  supporting  tissue  of  von  Szily,  217). 

The  primordium  of  the  lens  now  begins  (at  a  gr.  1.  of  4  mm:  Keibel, 
116;  Seef  elder,  203)  over  the  summit  of  the  optic  vesicle  in  the  form 
of  a  thickening  of  the  ectoderm  (lens  plaque),  and  soon  sinks  into  a 
depression. 

At  the  same  time,  but  independently,  the  neighboring  already  thick- 
ened, distal  part  of  the  primary  optic  vesicle  becomes  concave  (convex 
brainward);    the  primary  optic  vesicle  begins  to  invaginate. 

The  much-used  expression,  "mvagination,"  is  not  correct.  The  distal  portion  of 
the  primary  optic  vesicle  does  not  actually  grow  inward,  but  the  margins  grow  out 
over  the  summit,  and,  indeed,  from  above  (dorsal)  and  from  the  sides. 


THE  EMBRYONAL  AND  FETAL  DEVELOPMENT  193 

Therewith  begins  the  transition  of  the  primary  into  the  secondary 
optic  vesicle  or  the  optic  cup  (PI.  IX,  7).  The  optic  cup  has  a  double 
wall;  that  portion  of  the  primary  optic  vesicle  which  is  not  invaginated 
forms  the  outer  leaf  (a),  the  invaginated  distal  portion,  the  inner  leaf  (/) 
of  the  optic  cup.  Its  lumen  (a)  has  the  form  of  a  cleft  and  is  connected 
with  the  lumen  of  the  forebrain  (F)  through  the  wide  lumen  of  the  pedicle 
of  the  optic  vesicle  (5).  The  cavity  of  the  optic  cup  arising  through  the 
invagination  shelters  the  lens  primordiiim  (L)  and  is  open  laterally,  i.e., 
toward  the  ectoderm,  and  below. 

Meanwhile,  small  vessels  of  a  capQlary  nature  have  developed  in  the 
mesoderm  surrounding  the  optic  cup. 

To  the  extent  to  which  the  lens  primordium  sinks  into  a  tiny  sac,  the 
optic  cup  also  deepens  and  more  and  more  surrounds  the  lens  primordium. 
The  opening  of  the  optic  cup  thereby  continues  to  differentiate  itself  more 
plainly  into  a  laterally  directed  rounded  portion  (the  primitive  pupil) 
and  into  a  downward-directed  cleft  (fetal  cleft,  optic  fissure),  which  at 
first  ends  at  the  beginning  of  the  pedicle  of  the  optic  vesicle. 

The  primordium  of  the  lens  is  now  surrounded  on  all  sides  by  the 
margins  of  the  optic  cup  and  separated  from  this  by  the  engirdling  meso- 
derm; the  mesoderm  borders  the  lens  primordium  below,  only  in  the 
region  of  the  fetal  cleft. 

There  is  only  a  narrow  interspace  between  the  lens  primordium 
and  the  inner  leaf  of  the  optic  cup,  and  this  is  filled  out  by  the  primitive 
vitreous.  This  is  nothing  more  than  that  layer  between  the  ectoderm 
and  the  optic  vesicle  present  immediately  before  the  invagination  of  the 
lens;  it  consists,  therefore,  of  von  Szily's  embryonal  supporting  tissue, 
which,  to  be  sure,  is  not  completely  free  of  cells  at  any  stage. 

The  borders  of  the  fetal  cleft  approach  each  other  and  narrow  the 
cleft  more  and  more.  Thereby  an  extension  of  the  mesoderm  becomes 
more  and  more  plainly  separated  off  and  presses  in  through  the  fetal 
cleft  into  the  cavity  of  the  optic  cup.  A  small  vessel,  a  branch  of  a  ring 
vessel,  develops  in  this  process  at  the  border  of  the  cup.  The  newly 
formed  vessel  ends  blind  behind  the  little  lens  sac  at  a  gr.  1.  of  5  mm 
(Seefelder,  199),  (at  one  of  7  mm  1.  according  to  Elze,  56);  by  the  twenty- 
eighth  day  it  extends  through  the  entire  optic  cup,  according  to  Calderaro 
(29),  and  is  united  with  the  vessels  of  the  region  behind.  This  is  the 
primordium  of  the  inner  vessel  system — the  primitive  arteria  hyaloidea — 
and  the  annular  vessel  at  the  border  of  the  cup  is  the  primordium  of  the 
circulus  iridis  major. 

At  a  gr.  1.  of  6.5  mm  (Keibel,  116),  the  little  lens  sac  has  been  closed  off 
into  a  lens  vesicle,  the  rounded  lumen  of  which  contains  some  descjuamated 
cells.     The  lens  vesicle  at  first  remains  connected  with  the  ectoderm. 


194  ANATOMY  AND  HISTOLOGY  OF    IIIK  Hl'.MAN  K^•|•;U\LL 

At  a  gr.  1.  of  8 . 5  mm  the  closure  of  the  optic  cleft  Ijcgins  (Soefelder, 
203) ;  the  mesoderm  between  the  edges  of  the  cleft  disapi)ears,  and  these 
fuse  together,  and,  indeed,  each  leaf  of  the  optic  cup  by  itself,  so  that  there- 
after not  even  the  slightest  trace  of  this  union  remains.  The  closure  of 
the  optic  cleft  begins  in  the  middle  and  j)roceeds  from  there  forward  and 
backward.  The  primitive  pupil  is  thereby  closed  off  into  a  round  open- 
ing directed  toward  the  ectoderm  (therefore  at  first  still  lateral,  later 
toward  the  front).  At  first,  however,  there  is  no  closure  at  the  posterior 
end  of  the  optic  fissure. 

At  a  gr.  1.  of  9.75  mm  (Tandler,  219)  the  lens  vesicle  becomes  com- 
pletely constricted  off  from  the  ectoderm,  and  its  lumen  begins  to  narrow 
from  behind  (through  elongation  of  the  epithelial  cells  concerned).  The 
mesoderm  between  the  ectoderm  and  the  lens  vesicle  now  also  begins 
to  grow  in,  i.e.,  the  formation  of  the  primitive  cornea  begins. 

In  this  stage  (PI.  IX,  8)  the  pedicle  of  the  optic  vesicle  (S)  is  always 
still  very  short  and  thick,  but  its  lumen  is  already  considerably  narrower. 
A  well-developed  extension  of  the  mesoderm  passes  into  the  cavity  of  the 
cup  at  the  transition  of  the  pedicle  into  the  optic  cup:  the  fissure  in 
the  cup  is  here  still  wide  open  (Mf).  This  extension  (the  primordium 
of  the  inner  vessel  system)  is,  moreover,  united  with  the  neighboring 
mesoderm  at  the  border  of  the  cup,  although  only  below  (ventral),  and 
the  border  of  the  cup  still  has  a  notch  at  this  place.  In  between,  how- 
ever, the  optic  fissure  is  closed,  and,  therefore,  the  cleft-like  lumen  of  the 
optic  cup  {A)  is  continuous.  The  cavity  of  the  optic  cup  is  for  the  greater 
part  taken  up  by  the  lens  vesicle  (L) ;  ventral  to  this  lies  the  process  of 
mesoderm,  and  the  remnant  of  the  cavity  is  filled  out  by  primitive 
vitreous  (G).  Between  the  border  of  the  cup  and  the  ectoderm,  mesoderm 
is  everywhere  found,  and  this  presses  in  like  a  wedge  between  the  lens 
vesicle  and  the  ectoderm  (H). 

It  is  difficult  to  give  a  comprehensive  description  of  the  further 
development  of  the  eye;  I,  therefore,  prefer  now  to  treat  the  individual 
portions  of  the  optic  primordium  separately. 

The  pedicle  of  the  optic  vesicle  becomes  the  optic  nerve.  It  now 
rapidly  grows  in  length  (Seef elder,  203).  The  suggestion  of  invagination, 
which  up  to  this  time  was  present  at  the  anterior  end,  deepens  to  a  plain 
furrow  and  extends  farther  backward.  The  pedicle  of  the  optic  vesicle  still 
consists  of  epithelial  cells,  however,  and  maintains  the  primary  lumen. 

At  a  length  of  14  to  15  mm  the  first  nerve-fibers  appear  in  the  periph- 
ery of  the  pedicle  of  the  optic  vesicle,  and  the  lumen  narrows.  At  23  mm 
length  the  pedicle  of  the  optic  vesicle  is  solid;  as  a  result,  the  furrow  is 
closed  and  the  proximal  portion  of  the  arteria  hyaloidea  is  closed  into  the 


THE  EMBRYONAL  AND  FETAL  DEVELOPMENT  195 

axis  of  the  pedicle.  The  connection  of  the  arteria  hyaloidca  with  the 
neighboring  mesoderm  persists  only  at  the  posterior  end  of  the  furrow, 
where  the  central  vessels  enter  later. 

The  nerve-fibers  sprout  in  from  the  optic  cup;  at  the  same  time  the 
train  of  epithelial  cells  becomes  spaced  apart  and  forms  a  syncytium,  i.e., 
an  area  of  protoplasmic  framework  permeated  with  cell-nuclei  (Krueck- 
mann,  124).  The  glia  fibers  develop  out  of  the  protoplasmic  processes, 
the  glia  cells  out  of  the  nucleated  portion  of  the  syncytium. 

The  primordium  of  the  vena  centralis  is  first  visible  at  a  length  of  54  mm 
(Seefelder,  201).  The  connective-tissue  septa  grow  in  with  the  blood- 
vessels out  of  the  neighboring  mesoderm;  the  latter  also  furnishes  the 
optic-nerve  sheaths. 

The  outer  leaf  of  the  optic  cup  {a)  develops  into  the  stratum  pigmenti. 

Pigmentation  begins  even  very  earl}-  (at  a  gr.  1.  of  7  mm,  Elze,  56), 
and,  indeed,  first  in  the  equatorial  regions  (Lauber,  137).  Yet  in  this 
respect  there  are  great  individual  differences.  At  9.75  mm  gr.  1.  (PI. 
IX,  8)  the  whole  outer  leaf  is,  indeed,  sparsely  but  uniformly  pigmented, 
yet  still  in  several  layers. 

At  19  mm  length  (Dedekind,  37)  the  posterior  part  of  the  optic  vesicle 
has  already  become  one  layer,  which  still  thickens,  however,  toward  the 
border  of  the  cup.  The  single-layer  portions  consist  chiefly  of  flat  endo- 
thelial-like  cells;  they  attain  their  cylindrical  form  first  after  birth 
(Seefelder,  208). 

The  inner  leaf  of  the  optic  cup  becomes  differentiated  into  the  retina 
but  also  contributes  to  the  formation  of  the  primitive  vitreous. 

Even  at  the  beginning  of  the  invagination  of  the  primary  optic  vesicle 
(PL  VIII,  7),  the  distal  portion  of  its  wall  (/)  is  characterized  by  a  greater 
thickness,  and  a  nuclear-free  layer  is  found  along  the  basal  surface  (that 
turned  toward  the  lumen) — the  marginal  film  of  His. 

This  m.arginal  film  develops  further  into  an  increasingly  plain  syncyt- 
ium, the  primordium  of  the  supporting  tissue.  At  a  length  of  11.3cm 
a  layer  difl'erentiates  itself  off  from  the  heretofore  undift"erentiated  thickly 
disposed  nuclei  inward.  The  nuclei  in  this  are  not  so  thickly  disposed — the 
primordium  of  the  ganglion-cell  layer ;  it  soon  becomes  much  thickened. 

The  formation  of  the  ganglion  cells  begins,  as  does  the  differentiation 
of  the  retina,  in  general,  in  the  tempero-inferior  quadrant,  and  this  area 
later  becomes  the  region  of  the  fovea.  Differentiation  progresses  periph- 
eralward  from  here. 

The  first  nerve-fibers  appear  in  the  border  film  at  a  length  of  13  to 
14  mm;  at  55  mm  the  dendrites  of  the  ganglion  cells  and  their  diplosomes 
and  at  the  same  time  the  inner  nuclear  layer  dift'erentiate  themselves. 


196  ANATOMY  AND  HISTOLOGY  OF  ']TIK  HUMAN  EYEBALL 

From  this  time  on  the  thickness  of  the  ganglion-cell  layer  again 
decreases  as  a  result  of  the  increase  of  the  retina  in  surface  expanse;  its 
greater  thickness  is  maintained  only  in  the  region  of  the  differentiation 
center  (primordium  of  the  area  centralis). 

A  cell-layer  first  appears  on  the  side  of  the  retina  which  was  originally 
free  (now  lying  on  the  pigment  epithelium)  at  the  end  of  the  third  month; 
this  is  the  primordium  of  the  outer  nuclear  layer.  At  the  fifth  month  the 
rods  appear  as  small  caps  projecting  over  the  membrana  limitiDis  externa; 
the  diplosomes  lie  in  them,  and  at  each  cell  a  fine  thread  (the  outer  thread) 
goes  into  the  pigment  epithelium  from  the  diplosome. 

The  amacrin  cells  separate  away  from  the  nuclei  of  the  Mueller's  fibers 
in  the  fifth  month  in  the  primordium  of  the  area  centralis,  and  there  arises 
an  intervening  layer  (Chievitz;  transitory  fiber  layer,  32).  This  layer  later 
again  disappears,  but  traces  of  it  are  often  found  in  extrauterine  life. 

At  the  end  of  the  sixth  month  that  reduction  of  the  cerebral  layer 
which  leads  to  the  formation  of  the  fovea  centralis  begins  in  the  middle 
of  the  area  centralis.  The  distance  between  the  fovea  and  the  papilla 
is  already  as  great  as  in  the  adult  eye. 

Although  up  to  this  time  the  development  in  the  area  centralis  pre- 
cedes that  of  the  rest  of  the  retina,  it  now  falls  behind,  at  least  in  respect 
to  the  devlopment  of  the  neuroepithelium,  and  even  at  birth  the  fovea 
is  not  yet  completely  developed  (cf.  chap.  xvii). 

The  retinal  vessels  sprout  out  from  the  portion  of  the  arteria  Iiyaloidca 
(later  the  art.  centralis  retinae)  inclosed  in  the  optic  nerve  at  a  length  of 
10  cm  (Versari,  231),  or  at  the  beginning  of  the  fourth  month  (Seefelder, 
201),  and  at  once  press  into  the  nerve-fiber  layer  of  the  retina,  in  which 
they  gradually  broaden  out  farther.  The  vessel  system  is  only  com- 
pletely developed  in  the  eighth  month.  A  membrana  vasculosa  retinae, 
therefore,  does  not  exist  at  any  time  in  man. 

We  have  left  the  lens  primordium  as  a  hollow  epithelial  vesicle  with 
somewhat  elongated  cells  in  the  posterior  wall  (PI.  IX,  8).  By  further 
elongation  of  their  axes  these  posterior  cells  grow  out  to  lens  fibers  and  so 
fill  out  the  lumen  of  the  lens  vesicle  (at  a  length  of  13  mm  according  to 
Brueckner,  28).  From  there  on  the  new  formation  of  fibers  is  limited  to 
the  equatorial  portions  of  the  vesicle.  The  youngest  fibers  at  the  periph- 
ery are  strongly  concave  toward  the  equator;  this  bowing  is  gradually 
lost  toward  the  center,  where  a  purer  sagittal  direction  of  the  fibers  is 
present  (central  fibers  of  Rabl,  175).  Since  all  the  fibers  are  nucleated, 
the  nuclear  bow  extends  through  the  entire  lens  (PI.  IX,  9). 

At  a  length  of  51  mm  (collection  of  the  I.  Anatomic  Institute  of 
Vienna)  there  is  already  manifest  a  tendency  to  concentric  stratification, 


THE  EMBRYOXAL  AND  FETAL  DEVELOPMENT  197 

i.e.,  the  fibers  undergoing  development  are,  as  heretofore,  strongly  con- 
cave toward  the  equator;  the  middle  fibers,  however,  show  a  convexity 
toward  the  equator  (transition  fibers  of  Rabl),  and  only  the  central  fibers 
are  strictly  sagittal. 

At  60  mm  length  (collection  of  the  I.  Anatomic  Institute  of  Vienna)  a 
plain  cortex  of  concentric  layers  of  fibers  with  three-rayed  lens  stars  is 
already  present.  The  form  of  the  lens  is  still  almost  spherical.  The 
further  growth  then  continues  as  in  the  adult,  with  only  this  difference, 
that  the  number  of  undeveloped  lens  fibers  is  much  greater  and  their 
concavity  more  outspoken. 

At  the  beginning  (at  the  time  of  the  invagination)  the  cells  of  the  lens 
primordium  also  contribute  to  the  make-up  of  the  primitive  vitreous,  and, 
indeed,  by  the  formation  of  conical  basal  processes  (lens  cone  of  von 
Lenhossek,  140) ;  this  subsequently  breaks  up  into  fine  fibers  which  unite 
with  similar  fine  filaments  proceeding  from  the  inner  leaf  of  the  optic  cup. 
The  prevailing  direction  of  the  fibers  is  radial  (embryonal  supporting 
tissue  of  von  Szily). 

The  lens  vesicle  is  soon  closed  off  from  the  neighboring  structures  by 
a  cuticula  (later  the  lens  capsule),  and  thereby  sacrifices  any  further 
influence  on  the  development  of  the  vitreous. 

The  mesodermal  process  which  presses  in  through  the  optic  cleft 
grows  about  the  lens  on  all  sides  and  forms  the  capsula  perilenticularis 
(Cirincione,  2iZ)-  Vessels  appear  in  this  in  the  seventh  week,  and  at  the 
same  time  the  undifferentiated  mesoderm  disappears ;  in  the  ninth 
week  the  capsula  perilenticularis  goes  over  into  the  tunica  vasculosa 
lentis  (PI.  IX,  9,  Tv),  which  consists  only  of  vessels.  Numerous  branches 
of  the  arteria  hyaloidea  sprout  out  to  the  sides  and  toward  the  front,  and 
the  latter  form  the  tunica  vasculosa  lentis,  a  net  of  vessels  which  wholly 
surrounds  the  posterior  half  of  the  lens  and  unites  with  the  primordium 
of  the  outer  vessel  system  along  the  entire  border  of  the  cup. 

Up  to  a  SS  of  20  mm  (second  month)  all  the  vessels  ha^•e  the  same 
caliber,  according  to  Calderaro  (29).  A  trunk  (the  arteria  hyaloidea 
proper)  then  dift'erentiates  itself,  and  courses  from  behind  (as  the  branch 
of  the  art.  ophthalmica)  through  the  pedicle  of  the  optic  vesicle  and  the 
axis  of  the  primitive  vitreous.  It  gives  off  lateral  branches  in  the  vitreous 
(vitreous  vessels)  and  divides  into  several  branches  in  the  neighborhood 
of  the  lens;  these  carry  the  blood  to  the  tunica  vasculosa  lentis.  The 
drainage  in  front,  at  the  border  of  the  cup,  proceeds  into  the  outer  vessel 
system,  and  this  place  is  called  the  isthmus  (PI.  IX,  9,  /),  since  the  border 
of  the  optic  cup  almost  always  lies  close  to  the  lens. 

At  31  mm  length   (Seef elder,   203)   a  conical  or  rod-form  structure 


198  ANATO^n-  AM)  HISTOLOGY  OF  THK  Hl'MAX  F.YEBALL 

develops — zaffo  prepapillare  (Calderaro,  29),  glial  mantle  (Seefelder,  203) 
— at  the  place  where  the  arteria  hyaloidea  passes  out  of  the  optic  nerve 
into  the  vitreous.  According  to  Seefelder,  it  consists  of  glia  cells,  arranged 
in  two  layers,  and  when  fully  developed  may  attain  a  length  of  2  mm 
(Calderaro).  This  glial  mantle  fills  out  the  excavation  arising  at  the 
entrance  of  the  optic  nerve  occasioned  by  the  spreading  apart  of  the 
optic-nerve  fibers  (Seefelder,  200).  Fibrillae  are  set  into  the  surface  of 
the  glial  mantle;  these  course  straight  as  a  string  to  the  lens  and  there 
spread  apart  like  a  crater  (central  vitreous  body  of  Retzius). 

The  disappearance  of  the  inner  vessel  system  begins  at  the  fifth  month 
with  the  vessels  of  the  vitreous  proper;  the  circulation  in  the  arteria 
hyaloidea  ceases  in  the  sixth  month,  according  to  Calderaro;  in  the  seventh 
month  it  is  transformed  into  a  filament,  and  this  disappears  also  between 
the  eighth  and  ninth  month.  According  to  Seefelder,  however,  the  arteria 
hyaloidea  carries  blood  much  longer.  With  the  disappearance  of  the 
arteria  hyaloidea  and  its  glial  mantle  the  excavation  again  opens. 

That  which  later  becomes  the  central  canal  appears  to  be  identical 
with  the  cavity  of  the  central  vitreous  body  of  Retzius,  if  I  have  under- 
stood Seefelder  (203)  correctly.  However,  according  to  the  view  of  this 
author,  it  also  becomes  filled  out  with  vitreous  tissue  and  Seefelder,  like 
Wolfrum,  therefore,  denies  the  existence  of  a  central  canal  (p.  149). 

The  tunica  vasculosa  lentis  undergoes  regression  at  the  same  time  as  the 
arteria  hyaloidea.  Only  that  portion  of  this  artery  lying  in  the  optic 
nerve  persists,  and  from  this  time  on  supplies  only  the  retinal  vessel 
system;    it  becomes  the  arteria  centralis  retinae. 

After  the  development  of  the  lens  capsule  the  predominantly  radial 
fibers  of  the  primitive  vitreous  are  united  rather  with  the  retinal  primor- 
dium,  and,  indeed,  with  the  marginal  film.  The  formation  of  further 
radial  fibers  proceeds  from  this  (Wolfrum,  239) ;  these  are  then  united  by 
cross-anastomoses.  Protoplasmic  unions  with  the  vitreous  vessels,  which, 
in  general,  are  pure  endothelial  tubes,  do,  however,  also  come  about. 
The  cross-anastomoses  at  length  acquire  predominance  in  the  fundus  and 
so  go  over  gradually  into  the  permanent  structure. 

Finally,  the  retina  also  becomes  closed  off  from  the  vitreous  by  a 
cuticulum  (the  subsequent  mcmbr.  limitans  interna).  However,  the  forma- 
tion of  radial  fibers  proceeds  farther  in  the  region  of  the  pars  coeca  (see 
p.  199),  which  has  arisen  meanwhile,  so  that  the  definitive  vitreous  is 
mainly  connected  with  the  ciliary  epithelium  and  appears  to  proceed  out 
of  this  (vitreous  basis). 

The  formation  of  these  zonula  fibers  from  the  cells  of  the  pars  coeca 
goes  on  in  a  way  similar  to  the  development  of  the  vitreous  fibers.     They 


THE  EMBRYOXAL  AND  FETAL  DEVELOPMENT  199 

are  only  to  be  distinguished  from  the  fibers  of  the  vitreous  by  the  fact 
that  they  are  larger  and  form  no  cross-anastomoses  (Wolfrum,  239). 

The  \ie\vs  of  the  embr\-ologists  have  undergone  a  significant  change  in  the  last 
decades  with  respect  to  the  genesis  of  the  \dtreous.  Previously  the  vitreous  was  held 
to  be  a  mesodermal  structure.  Now,  with  a  few  exceptions,  the  trend  of  the  \'iews  is 
that  the  mesoderm  forms  only  the  vitreous  vessels,  that,  however,  the  framework  of 
the  \'itreous  is  of  ectodermal  origin.  In  respect  to  the  finer  details  the  \-iews  are  still 
very  much  at  \-ariance;  I,  myself,  have  mainly  followed  the  more  intermediate  views 
of  Koelliker,  von  Szily,  and  Wolfrum. 

Even  the  fetal  vitreous  is  very  poor  in  cells  (aside  from  the  vitreous 
vessels);  these  cells  are  explained  in  various  ways,  but,  as  it  appears,  the 
later  works  are  agreed  that  these  cells  form  no  essential  part  of  the  fetal 
\atreous. 

In  the  mesoderm  surrounding  the  optic  cup,  a  layer  of  capillary  vessels 
very  early  becomes  differentiated  off — one  lying  immediately  on  the  outer 
leaf  of  the  optic  cup  (the  primitive  choriocapillaris) .  At  a  SS  of  19  mm., 
according  to  Dedekind  (37),  there  is  a  thicker  layer  of  mesoderm  (the 
primordium  of  the  sclera)  (PI.  IX,  9,  S)  outside  this  capillary  layer,  and 
in  the  posterior  part  of  the  chorioidal  primordium  a  second  layer  of  larger 
vessels  is  already  demonstrable.  Likewise  the  vortex  veins  are  laid  out, 
as  well  as  the  long  posterior  ciliary  arteries,  the  temporal  of  which  appears 
as  a  direct  extension  of  the  arteria  ophthalmica. 

The  primitive  cornea  (PI.  IX,  8,  H)  differentiates  itself  in  a  similar 
way,  i.e.,  forms  out  of  that  layer  of  mesoderm  which  has  interposed  itself 
between  the  ectoderm  and  the  lens  vesicle.  This  layer,  which  in  the 
seventh  week  is  still  completely  undifferentiated,  divides  into  an  outer 
(anterior),  i.e.,  lying  immediately  under  the  ectoderm  avascular,  and  into 
an  inner  (posterior)  vascular  layer  (Seef elder,  216). 

The  anterior,  much  thicker  layer  (PI.  IX,  9,  H)  is  the  primordium 
of  the  corneal  stroma ;  the  posterior  layer  (IP)  is  pretty  thick  at  the  border 
of  the  cup,  much  thinner  in  the  middle,  and  is  best  designated  as  the 
lamina  irido-pupillaris  (Jeannulatos,  no).  The  border  of  this  portion  is 
formed  by  a  layer  of  regularly  arranged  cells  (primordium  of  the  endo- 
thelium) at  a  length  of  26  mm,  and  this  layer  ends  opposite  the  cup  border 
in  a  group  of  such  cells  (primordium  of  the  scleral  trabeculum). 

The  lamina  irido-pupillans  is  united  at  the  isthmus  (/)  with  the  tunica 
vasculo^a  lentis,  the  vessels  of  which  bend  about  the  border  of  the  optic  cup. 

Toward  the  end  of  the  third  month  the  pars  coeca  of  the  optic  cup  is 
laid  out,  i.e.,  the  border  of  the  optic  cup  grows  out  into  an  epithelial  fold, 
which  in  the  course  of  further  development  always  becomes  more  closely 
articulated  to  the  peripheral  portion  of  lamina  irido-pupillaris. 


200  ANATOMY  AND  HISTOLOGY  OF  THE  IIUMAN   KNI'BAl-L 

The  pars  coeca,  like  the  optic  cuj)  itself  has  two  leaves,  an  outer  and 
an  inner.  The  outer  leaf  is  intensely  pigmented  and  is  soon  disposed  in 
meridional  folds  (primordium  of  the  ciliary  processes),  while  the  inner 
unpigmented  layer  at  first  courses  smoothly  over  these  folds.  The  transi- 
tion area  of  the  pars  cocca,  or  the  forward  displaced  border  of  the  optic 
cup,  is  not  folded.  At  this  place  a  narrow  space  exists  between  the  two 
leaves  (analogous  to  the  lumen  of  the  primary  optic  vesicle,  or  possibly 
to  the  last  remnant  of  this  lumen,  the  ring  sinus  of  Szili,  216). 

At  the  beginning  (Szili,  216;  Lauber,  137)  or  toward  the  end  of  the 
fourth  month  (Seefelder,  204)  a  club-like  (or  possibly  better,  roll-like) 
thickening  appears  at  the  transition  place — the  primordium  of  the 
sphincter  pupillae.     Herewith  begins  the  development  of  the  iris  proper. 

The  sphincter  primordium  consists  of  ectodermal  cells,  derivatives 
of  the  transition  area,  in  part  also  of  the  outer  layer  (Juselius,  112). 
It  is  united  wholly  to  the  pars  coeca  (PI.  IX,  10,  Spli)  to  begin  with, 
but  imbeds  itself,  however,  in  further  development,  in  the  mesoderm. 
Yet  numerous  connections  with  the  pars  coeca  always  remain. 

According  to  Seefelder  (204),  the  unfolding  of  the  anterior  chamber 
begins  in  the  fifth  month,  and,  indeed,  first  in  the  region  of  the  iris  primor- 
dium. From  there  it  gradually  advances  toward  the  axis  of  the  eye, 
and  is  only  completed  in  the  sixth  month.  Still  its  peripheral  limit  is 
then  at  the  border  of  Descemet's  membrane. 

The  iris  angle  then  forms;  the  mesoderm  between  the  primordium  of 
the  scleral  trabeculum  and  that  of  the  iris  becomes  spaced  apart  and  trans- 
formed into  a  loose  framework,  the  uveal  framework  of  H.  Virchow  (234). 
This  framework,  therefore,  fills  out  the  entire  iris  angle  in  the  fetus; 
later  it  disappears  except  for  a  slight  remnant  at  the  periphery  of  the 
chamber  bay. 

A  complete  separation  of  the  corneal  primordium  and  its  endothelium, 
on  the  one  side,  from  the  lamina  irido-pupillaris,  on  the  other  side,  follows 
in  the  development  of  the  anterior  chamber.  The  further  development 
of  the  anterior  layer  is  quite  simple  (PI.  IX,  10):  the  ectoderm  becomes 
the  corneal  epithelium  (£),  the  mesoderm  becomes  the  corneal  stroma 
(C),  and  the  above-reported  cell-layer,  the  endothelium  (Z>);  this  latter 
at  first  consists  of  small,  quite  high  cells.  The  Descemet's  membrane 
arises  as  a  cuticulum  out  of  the  endothelium,  and  is  demonstrable  from 
the  fourth  month  on.  According  to  the  newest  investigations,  a  pre- 
corneal vessel  net  does  not  exist  (Hirsch,  103). 

The  lamina  irido-pupillaris  shows  a  thicker  zone  in  the  periphery 
covered  posteriorly  by  the  pars  coeca — the  primordium  of  the  iris  (/). 
The  vessels  of  the  tunica  vasculosa  lentis  (Ti^  enter  the  lamina  irido- 
pupillaris  at  the  inner  margin  of  the  iris  primordium. 


THE  EMBRYOXAL  AND  FETAL  DEVELOPMENT  201 

By  far  the  greater  remnant  of  the  lamina  irido-piipillaris  is  a  delicate 
membrane  consisting  mainly  of  vessels  and  called  the  pupQlary  membrane 
{Pm). 

With  the  further  growth  of  the  eye,  the  iris  continues  to  broaden  out 
along  with  the  contemporary  growth  of  the  pars  coeca,  and  the  impres- 
sion is  therefore  given  that  the  iris  grows  out  of  the  chamber  angle. 
In  fact  the  pupil  always  becomes  larger  with  the  age  of  the  fetus;  this 
is  only  due  to  the  fact  that  its  increase  in  size  does  not  keep  pace  with 
that  of  the  whole  eyeball. 

The  iris  primordium,  therefore,  consists  of  a  mesodermal  layer  (part 
of  the  lamina  irido-piipillaris)  and  two  ectodermal  layers,  i.e.,  the  anterior 
zone  of  the  pars  coeca.  The  mesoderm  of  the  iris  primordiitm  at  first 
goes  continuously  over  into  the  pupillary  membrane,  and  the  vessel 
system  of  the  iris  forms  a  continuum  with  that  of  the  pupillary  membrane. 
Furthermore,  the  vessel  system  of  the  iris  takes  up  the  drainage  of  the 
tunica  vasculosa  lentis.  The  circulation  in  the  pupillary  membrane  is, 
therefore,  independent  of  that  of  the  tunica  vasculosa  lentis.  Later,  the 
border  between  the  iris  and  the  pupillary  membrane  moves  back  onto 
the  anterior  surface  of  the  former,  probably  because  of  the  greater  develop- 
ment of  the  sphincter  pupillac  and  its  neighborhood  (Brueckner,  28). 

The  pars  uvealis  iridis  develops  out  of  the  mesodermal  layer  of  the 
iris  primordium;  stroma  cells  differentiate  themselves  in  it  in  the  fourth 
month,  the  anterior  border  layer  in  the  seventh  month  (Lauber,  137). 

At  first,  the  outer  leaf  of  the  pars  coeca  consists  of  high  cylindrical 
cells  ciliaryward,  of  low  cylindrical  cells  in  the  region  of  the  sphincter 
primordium  (Spli).  On  the  border  between  the  two  forms  of  cells, 
Michel's  spur  arises;  and  the  clump  cells  arise  through  detachments  from 
this,  and  from  the  pigment  spurs  lying  farther  forward. 

The  ciliary  zone  of  the  outer  leaf  becomes  differentiated  into  the 
dilatator  pupillae.  According  to  Heerfordt  (88),  the  bases  of  the  cells 
fuse  to  a  diffusely  pigmented  lamella  in  the  twenty-second  week.  In  the 
twenty-fourth  to  the  twenty-eighth  week,  pigment  disappears  from  this 
lamella,  and  fine  meridional  fibrillae  become  visible  in  it.  In  the  thirtieth 
to  the  thirty-second  week  bundles  of  fibrillae  become  separated  off  from 
one  another — the  subsequent  fibers  of  the  posterior  border  lamella. 
Meanwhile,  the  height  of  the  cells  gradually  decreases. 

The  inner  leaf  of  the  pars  coeca  takes  on  pigmentation  and,  indeed, 
progressively  ciliaryward  from  the  transition  area.  This  pigmentation 
has  attained  the  iris  root  at  a  length  of  19  cm  (Juselius,  112). 

The  ring  sinus  disappears  at  the  end  of  the  seventh  month,  according 
to  Szili,  yet  the  connection  of  the  two  leaves  remains  more  loose  at  this 


202  ANATOMY  AND  HISTOLOGY  OF  THE  HUMAN  EYEBALL 

place  than  in  the  ciliary  part  of  the  iris;  in  posterior  synechia  the  ring 
sinus  at  times  again  opens. 

The  pu[Mllary  membrane  persists  longer  than  the  tunica  vasculosa 
lentis  (Brueckner,  28).  Its  resorption  begins  in  the  eighth  month,  and, 
indeed,  iirst  in  the  center.  The  small  iris  circle  and  the  pupillary  cr>'pts 
form  after  the  disappearance  of  the  pupillary  membrane,  while  the  ciliary 
crypts  open  after  the  completion  of  the  regression  of  the  uveal  framework 
in  the  nin*^h  month. 

According  to  Seef elder  (204),  the  development  of  the  ciliary  body 
sets  in  at  the  end  of  the  third  month  by  a  folding  of  the  outer  leaf  of  the 
pars  coeca.  The  first  muscle  fibrillae  are  visible  in  the  meridional  por- 
tion of  the  ciliary  muscle  toward  the  end  of  the  fourth  month;  the  circular 
fibers  first  appear  at  the  end  of  the  sixth  month  with  the  opening  up  of  the 
chamber  bay. 

The  ciliary  processes  at  first  reach  much  farther  over  the  iris  (Taf. 
IX,  10,  Pc)  than  they  do  in  the  developed  eye.  Moreover,  to  begin 
with,  the  ciliary  body  consists  only  of  the  corona  cUiaris,  i.e.,  the  border 
of  the  retina  lies  at  the  posterior  ends  of  the  ciliary  processes  and  sends 
short  extensions  into  the  ciliary  valleys  (O.  Schultze,  198).  The  primor- 
dium  of  the  ciliary  muscle  at  first  extends  far  behind  the  border  of  the 
retina  {R). 

It  is  apparent,  therefore,  that  a  shifting  occurs  in  this  region  of  the 
eyeball  in  further  development.  The  ciliary  processes  shift  out  of  the  ter- 
ritory of  the  iris  into  that  of  the  ciliary  muscle  and  the  border  of  the 
retina  removes  itself  more  from  the  ciliary  processes.  The  backward 
displacement  of  the  ciliary  processes  cannot  well  be  an  actual  one,  for  its 
foundation  is  mesodermal  tissue;  the  form  only  of  the  ciliary  body  reaUy 
changes.  But  the  border  of  the  retina  actually  shifts  backward  in  its 
relation  to  the  posterior  end  of  the  ciliary  muscle.  Thereby  the  origi- 
nally short  projections  of  the  border  of  the  retina  become  drawn  out  to 
the  long,  sharp  teeth  of  the  or  a  serraia  (cf.  p.  88). 


As  already  reported,  the  optic  vesicle  is  laid  out  laterall\',  i.e.,  the 
primitive  pupil  looks  to  the  side.  After  the  closure  of  the  optic  cleft  the 
direction  of  the  eyes  changes  with  the  greater  development  of  the  skull; 
the  pupils  turn  more  toward  the  front,  and  that  which  was  the  posterior 
(caudal)  half  of  the  eye  in  the  primordium  becomes  the  lateral  (temporal) 
half  in  the  developed  eye. 

However,  no  notable  rotation  of  the  eye  about  its  axis  occurs,  for  the 
entrance  of  the  central  artery  into  the  optic  nerve  (the  place  correspond- 
ing to  the  posterior  end  of  the  optic  cleft)  also  lies  below  in  the  adult.    The 


THE  EYEBALL  OF  THE  NEWBORN 


203 


fovea  centralis  has,  therefore,  no  relation  to  the  optic  cleft,  wholly  aside 
from  the  fact  that  it  develops  at  a  much  later  period  of  fetal  life.  The 
physiologic  excavation  has  just  as  little  to  do  with  the  optic  cleft. 


CHAPTER  XVn.     THE  EYEBALL  OF  THE  NEWBORN 

(Text  Fig.  s) 

The  size  of  the  eyeball  varies  considerably  at  the  time  of  birth; 
according  to  E.  von  Jaeger  (108),  the  sagittal  diameter  varies  between  16 .  i 
and  19 . 1  mm.  The  mean  of  these,  and,  furthermore,  the  measurements  of 
Koenigstein(i2i),Merkel  and  Orr  (152),  Weiss  (235)  and  vonPflugk  (172), 
is  17.3  mm. 

The  form  of  the  eyeball  also  is 
subject  to  much  change ;  while  Weiss 
finds  the  equatorial  diameter  smaller 
than  the  sagittal,  Merkel  and  Orr,  as 
well  as  von  Pfiugk,  estimate  a  some- 
what higher  average  for  it.  All  are 
agreed,  however,  that  the  variation 
from  the  form  of  a  sphere  (cf.  p.  4)  is 
greater  in  the  newborn,  and  especially 
that  the  postero-temporal  part  is  more 
markedly  curved  out.  The  distance 
of  the  cornea  from  the  optic  nerve  is, 
therefore,  found  to  be  o .  3  to  o .  5  mm 
less  than  the  sagittal  axis  (Koenig- 
stein);  this  asymmetry  comes  out 
plainly  in  some  drawings  of  Weiss,  also  in  the  photographs  of  von  Pfiugk. 

According  to  von  Reuss  (181),  the  horizontal  diameter  is  usually 
9  mm,  according  to  Koenigstein,  10  mm.  The  cornea  is,  therefore, 
relatively  large;  its  relations  to  the  optic  axis  in  the  newborn  is  something 
like  1:1.8  (in  the  adult  something  like  1:2),  or  the  cornea  must  grow 
one-fifth  to  one-fourth,  the  optic  axis,  however,  at  least  one-third,  in 
order  to  attain  its  definitive  size. 

The  radius  of  curvature  is  given  as  6.59  by  von  Reuss,  as  7.3  by 
Merkel  and  Orr.  According  to  the  latter,  the  cornea  is  more  curved  at 
the  border  than  in  the  central  portions,  therefore,  a  relation  which  is 
exactly  the  opposite  of  that  in  the  adult  eyeball. 

Extensive  investigations  have  been  made  concerning  the  insertions 
of  the  eye  muscles  by  Weiss.  They  give  so  great  a  variation  in  the  posi- 
tion and  direction  of  the  lines  of  insertion,  that  I  must  limit  myself  to 


Text  Fig.  5. — Left  eyeball  of  the  newborn. 
Schematic  cross-section  closely  following  a 
photograph  of  von  Pflugk's.     Magnification  3. 


204  ANATOMY  AND  HISTOLOGY  OF  THE  HUMAN  EYEBALL 

reference  to  the  original  work.  Most  striking  and,  as  it  a])])ears,  pretty 
constant  is  the  relatively  slight  distance  from  the  cornea  at  which  the 
m.  rectus  medialis  is  inserted. 


The  sclera  has  its  greatest  thickness  at  the  posterior  pole,  its  least  in 
the  neighborhood  of  the  cornea  (E.  von  Hippel,  102).  The  thickness 
of  the  cornea  varies  a  great  deal  and  is  subject  to  the  influence  of  the 
fixation  fluid  perhaps  in  an  even  higher  grade  than  in  the  adult. 
Von  Hippel  found  a  maximum  of  i .  1 2  mm  in  fresh  eyes.  Still  greater 
thicknesses  (2  mm,  according  to  Hirschberg,  104)  are  possibly  due 
to  swelling. 

Bowman's  membrane  shows  the  same  thickness  as  in  the  adult  (17 
to  21  mu,  according  to  von  Hippel).  The  stroma  of  the  cornea,  like  the 
sclera,  is  much  richer  in  nuclei  than  in  the  adult  eye.  Descemet's  mem- 
brane, on  the  other  hand,  is  still  very  delicate  (2  mu). 

According  to  von  Hippel,  the  perichorioidal  space  is  entirely  absent; 
according  to  Merkel  and  Orr  it  is  absent  in  the  posterior  portion;  accord- 
ing to  Lange  (135),  however,  it  is  as  much  or  as  little  present  in  the 
posterior  segment  as  in  the  adult. 

Pigment  is  absent  in  the  stroma  of  the  uveal  tract  (but  not  the  cells 
which  will  later  bear  it);  at  the  most,  pigmented  chromatophores  are 
found  in  the  neighborhood  of  the  optic  nerve.  The  iris  also  is  almost 
always  gray,  therefore,  and,  indeed,  on  account  of  the  delicacy  of  the  pars 
uvealis,  a  pretty  dark  gray.  In  any  case,  the  pigmentation  of  the  anterior 
border  layer  sometimes  develops  in  the  first  days  of  life. 

The  stroma  of  the  uveal  tract,  furthermore,  shows  a  preponderance 
of  cells  over  collagenous  intervening  substance  (Gutmann,  83),  and,  there- 
fore, a  greater  richness  in  nuclei.  The  adventitia  of  the  vessels  is  very 
weakly  developed  and,  therefore,  the  marking  of  the  iris  is  more  delicate 
and  uniform.  The  pupil  is  pretty  narrow  and,  moreover,  does  not  per- 
mit of  maximal  dilatation. 

The  iris  angle  (chamber  bay)  is  sharp  and  narrow,  the  uveal  frame- 
work {ligamentum  pedinatum)  still  has  its  fetal  character  for  the  greater 
part,  i.e.,  it  fills  out  the  iris  angle  to  a  great  extent. 

The  ciliary  processes  still  reach  far  over  onto  the  posterior  surface  of 
the  iris,  they  are  thin  and  relatively  smooth,  i.e.,  the  lateral  folds  and 
bulgings,  as  well  as  the  little  warts  in  the  valleys,  are  only  suggested. 
The  ciliary  muscle  is  already  well  developed  and  even  the  various  types 
can  be  recognized  (Merkel  and  Orr,  Lange).  The  inner  surface  of  the 
orbiculus  ciliaris  looks  entirely  smooth  on  cross-section  (Taf.  IX,  11). 

The  pigment  epithelium  of  the  chorioidea  shows  a  uniform  develop- 


THE  EYEBALL  OF  THE  XEWBORX  205 

ment;  the  much  enlarged  cells  at  the  ora  serrata  fail.  On  the  other  hand, 
according  to  Kuhnt  (127),  some  larger  cells,  about  which  the  smaller 
ones  group  themselves  as  centers,  appear  at  pretty  regular  intervals. 
The  pigmentation  of  the  stratum  pigmenti  is  very  marked  in  comparison  to 
the  pigmentation  of  the  uveal  stroma,  indeed,  in  many  places  it  is  actually 
more  dense  than  in  the  adult.  Such  a  place  is  the  corona  ciliaris;  the 
summits  of  the  ciliary  processes  are  just  as  dark  a  brown  as  the  valleys; 
the  delicate  radiating  figure  from  which  the  corona  ciliaris  gets  its  name, 
is,  therefore,  not  visible  in  the  newborn.  Likewise  the  dilatator  piipillae 
appears  much  thicker  and  more  densely  pigmented. 

The  retina  is,  in  general,  well  developed,  to  be  sure,  yet  it  still  bears 
more  of  a  fetal  character  in  two  places,  namely,  the  region  of  the  fovea 
and  at  the  border. 

The  fovea  centralis  is  shallow  and  the  wall  about  it  barely  suggested. 
According  to  von  Hippel  (102),  all  the  layers  are  still  present  on  the  floor 
of  the  fovea — a  simple  layer  of  ganglion  cells  and  an  inner  nuclear  layer, 
somewhat  thinned  and  spaced  up,  to  be  sure,  but  yet  always  a  plainly 
distinguishable  layer.  On  the  other  hand,  the  external  nuclear  layer 
consists  only  of  one  layer  of  nuclei  and  the  cones  are  short,  thick,  and  few 
in  number  (Wolf rum,  241). 

The  distance  between  the  fovea  and  the  papilla  is  as  great  as  in  the 
adult. 

According  to  von  Hippel,  the  border  of  the  retina  shows  plainly 
formed  teeth,  although  they  are  not  as  long  as  in  the  adult  e}'e. 

The  border  of  the  retina  shows  an  especially  characteristic  picture  on 
meridional  section  (Taf.  IX,  11),  because  it  varies  much  from  that  of 
the  later  state.  In  particular,  the  retina  {R)  goes  over  into  the  ciliary 
epithelium  more  gradually.  At  the  place  where  the  inner  plexiform  layer 
ceases,  or  even  somewhat  farther  posterior,  the  two  nuclear  layers  fuse 
into  one,  and  out  of  these  fused  nuclear  layers  there  comes  a  tissue  which 
strikingly  recalls  the  first  stages  of  the  development  of  the  retina  in  the 
embryo.  This  zone  contains  numerous  uniform  oval  superimposed  nuclei 
and  to  the  inside  a  very  narrow  "border  film."  It  thins  very  gradually 
into  the  ciliary  epithelium  (C£),  which  appears  very  much  more  uniform 
and  lower  than  in  the  adult  eye. 

The  border  portion  of  the  retina  and  this  transition  zone  ele\-ate  them- 
selves very  readily  from  the  pigment  epithelium  and  then  form  a  fairly  high 
and  sharp  circular  fold  {F),  which  springs  inward  toward  the  vitreous  and 
somewhat  forward;  this  has  often  been  called  Lange's  fold  because  this 
author  (134)  has  made  remark  upon  its  regular  occurrence.  Yet  it  is  to 
be  looked  upon  as  an  artificial  product,  as  Lange  himself  concedes  (135). 


2o6  ANATOMY  AND  HISTOLOGY  OF  THE  HUMAN  EYIvBAlT. 

The  border  of  ihc  retina  always  lies  still  farther  forward,  about  at  the 
posterior  end  of  the  ciliary  muscle;  the  orbicitliis  ciliaris  is,  therefore, 
strikingly  short  (1.4  mm). 

The  pigment  epithelium  of  the  iris  shows  a  relatively  slight  pigmenta- 
tion, especially  toward  the  ciliary  border;  its  cells  are  small  and  the 
circular  furrow  system  is  absent.  The  sphincter  pnpillae  is  as  broad  as 
in  the  adult,  but  thinner. 

The  intraocular  end  of  the  optic  nerve  often  shows  a  plain  excavation 
— not  simpl}^  a  vessel  funnel  but  an  excavation  with  a  lateral  (elbow- 
formed)  transposition  of  the  nerve-fiber  bundles.  Remnants  of  the  glial 
mantle  of  the  arteria  hyaloidea  are  at  times  still  present.  The  diameter  of 
the  chorioidal  foramen  (the  ophthalmoscopic  papilla)  measures  about  i  mm. 
The  optic-nerve  fibers  behind  the  lamina  cribrosa  are  still  unmeduUated. 

The  vitreous  is  of  a  more  uniform  consistence;  the  border  layers  are 
only  weakly  developed. 

The  zonula  consists  of  numerous  fine  fibers;  moreover,  zonula  fibers 
are  given  ofi"  even  from  the  posterior  chamber  angle,  indeed,  even  from 
the  posterior  surface  of  the  iris,  as  far  as  the  ciliary  processes  reach.  Yet 
there  fail  any  of  the  divisions  characteristic  of  later  periods  of  life;  the 
fibers  are  uniformly  distributed  and  of  uniform  size. 

The  statements  concerning  the  form  and  the  diameter  of  the  lens  read 
very  differently.  In  the  text  and  drawing  (Text  Fig.  5)  I  hold  myself  to 
the  photograph  of  von  Pflugk  (172),  since  the  method  of  this  author  offers 
the  assurance  that  the  relations  of  fresh  cadaver-eyes  are  best  conserved. 

According  to  von  Pflugk,  the  form  of  the  lens  in  the  newborn  is  not 
so  fundamentally  different  from  that  of  the  adult  as  given  by  the  older 
statements.  The  equatorial  diameter  amounts  to  6.6  to  7  mm,  the 
thickness  to  3.4  to  4  mm.  The  radius  of  curvature  of  the  anterior 
surface  is  5  mm,  that  of  the  posterior  4  mm.  The  anterior  surface 
flattens  somewhat  toward  the  ecjuator,  the  posterior  surface  shows  a 
slight  concavity  in  the  neighborhood  of  the  ec|uator;  the  equator  itself 
is  pretty  sharp.     The  border  of  the  lens  is  smooth  and  without  crenations. 

The  lens  capsule  is  delicate;  only  the  thickening  at  the  periphery 
of  the  posterior  capsule,  which  even  Becker  (18)  observed,  is  strongly 
marked  (cf.  the  table  on  p.  165)  and  is  especially  striking  on  account  of 
the  delicacy  of  the  remaining  portion  of  the  lens  capsule.  The  nuclear 
bow  of  the  lens  is  still  long  and  rounded,  the  nuclei  reach  to  a  depth  of 
at  least  o .  4  mm.  The  central  portions  of  the  lens,  already  characterized 
by  a  somewhat  greater  density,  are  without  nuclei. 

On  account  of  the  strong  vaulting  of  the  lens  the  anterior  chamber 
is  pretty  shallow  (2 .3  to  2  .  7  mm,  according  to  von  Pflugk). 


EXTRAUTERINE  DEVELOPMENT  AND  GROWTH  OF  EYEBALL   207 


CHAPTER  XVHL   THE  EXTRAUTERINE  DEVELOPMENT  AND 
GROWTH  OF  THE  EYEBALL 

According  to  the  curve  drawn  by  Weiss  (235),  the  eyeball  grows  most 
rapidh-  in  the  first  years  of  life,  then  more  slowly.  From  the  fourteenth 
year  of  life  on  there  is  again  a  somewhat  greater  growth  up  into  the  twenties. 
The  growth  of  the  eye  keeps  pace  with  the  growth  of  the  brain;  in  its 
whole  period  of  growth  the  eye  grows  3.25  times,  the  brain  3  .  76  times — ■ 
the  body,  on  the  other  hand,  21 .36  times. 

The  rapid  growth  at  the  beginning  mainly  concerns  the  formation 
of  the  anterior  segment.  Von  Reuss  (181)  has  shown  that  the  average 
diameter  of  the  cornea  in  children  between  the  first  and  sixth  years  is 
not  much  less  than  the  average  in  the  adult.  This  would  therefore  indicate 
that  the  cornea  has  attained  almost  its  complete  size  in  the  course  of  the 
first  year  of  life.  According  to  the  table  published  by  Grod  (77),  it 
attains  this  in  the  second  year  of  life. 

While  the  cornea  thereby  grows  1.25  times,  the  neighboring  scleral 
zone,  i.e.,  the  zone  between  the  lines  of  the  insertion  of  the  recti  muscles 
and  the  border  of  the  cornea,  broadens,  in  general,  i  .358  to  i  .374  times 
(on  the  average),  according  to  Weiss,  and  the  completed  size  of  this 
segment  is  at  times  attained  even  in  early  childhood. 

Only  the  nasal  portion  of  this  zone,  i.e.,  the  interval  between  the 
insertion  of  m.  red.  nicdialis  and  the  border  of  the  cornea  shows  a  much 
greater  growth,  for  this  distance  is  on  the  average  i  .629  times  greater  in 
the  adult.  It  appears  that  this  greater  growth  sets  in  first  in  later  child- 
hood, yet  the  number  of  the  cases  is  still  much  too  small  to  cover  the 
great  indi\"idual  differences,  despite  the  comprehensive  investigations  of 
Weiss  concerning  this  circumstance. 

It  has  already  been  emphasized  that  the  distance  between  the  fovea 
centralis  and  the  papilla  is  as  great  in  the  newborn  (and  even  in  the  last 
months  of  fetal  life)  as  in  the  adult.  This  portion  of  the  wall  of  the  bulb, 
at  least  that  which  concerns  the  retina  and  the  pigment  epithelium  and 
probably  the  chorioidea  also,  does  not  grow  any  more,  therefore. 

The  last  increase  of  growth  from  the  fourteenth  year  of  life  on  appears 
mainly  concerned  with  the  enlargement  of  the  posterior  segment. 


As  we  now  turn  to  the  finer  anatomic  and  histologic  developmental 
processes  and  attempt  to  arrange  these  chronologically,  the  development 
of  the  medullary  sheaths  in  the  optic  nerve  must  be  considered  first.  At 
the  very  latest  this  is  completed  in  three  weeks  (Bernheimer,  22).  On 
this  account  alone  this  process  must  receive  special  attention,  because 


2oS  ANATOMY  AND  HISTOLOGY  OF  THE  HUMAN  EYEBALL 

it  shows  how  the  conditions  of  life  as  changed  by  birth  affect  develop- 
ment: it  is  the  light  which  favors  the  development  of  the  meduUary 
sheaths.  Prematurely  born  babes  who  have  lived  some  time  extra- 
uterine show  farther  advanced  sheath  development  than  fetuses  of  the 
same  age  which  have  remained  in  utero,  for  example. 

The  development  of  the  fovea  centralis  requires  a  somewhat  longer 
time.  After  four  weeks  a  plain,  steep-sided  depression  has  formed  (von 
Hippel,  102),  and  the  cerebral  layer  is  so  far  reduced  in  the  center  that  one 
can  no  longer  recognize  a  stratification.  But  the  outer  nuclear  layer  is 
always  still  poor  in  nuclei  and  the  cones  are  short.  The  fovea  first 
attains  its  full  development  months  after  birth  (Wolfrum,  241). 

The  perichorioidal  space  must  open  up  very  soon  after  birth.  At 
least  Elschnig  (54)  has  shown  that  a  strong  contraction  of  the  ciliary 
muscle  occurs  even  in  the  newborn,  although  it  is  still  without  plan,  and 
this  is  probably  not  conceivable  without  an  opening  of  at  least  the  anterior 
part  of  the  perichorioidal  space. 

As  already  stated,  the  cornea  concludes  the  greater  part  of  its  growth 
in  the  course  of  the  second  year  of  life.  The  diameter  of  the  cornea 
does  not  essentially  change;  later  the  radius  of  curvature  alone  is 
somewhat  increased.  According  to  von  Reuss  (181),  in  a  5-  to  6-year- 
old  child  this  is  7.36  mm  on  the  average,  increases  by  the  twelfth  year 
to  7.45  mm,  and  at  the  time  of  puberty  nearly  attains  the  average  size 
for  the  adult. 

With  the  increase  in  size  of  the  cornea,  the  fibrillar  intermediary 
substance  develops  more  and  more,  so  that  the  nuclear  richness  decreases. 
No  further  changes  appear  in  Bowman's  membrane.  Descemet's  mem- 
brane soon  attains  the  thickness  of  5  mu,  and  maintains  this  throughout 
the  whole  of  childhood.  Its  periphery  is  still  smooth  at  first.  I  have 
seen  the  earliest  suggestion  of  warts  in  the  ninth  year  of  life. 

The  iris,  too,  undergoes  further  development  during  this  period.  The 
difference  between  the  ciliary  and  the  pupillary  zone,  which  is  pretty 
indistinct  in  the  newborn,  comes  out  plainly  in  the  course  of  the  first 
half-year  of  the  extrauterine  life.  The  definitive  iris  color,  however,  needs 
a  longer  time  for  its  complete  development.  In  2-year-old  children  one 
sees  even  a  well-developed  adventitia  in  the  vessels  of  the  iris,  large  crypts, 
and  a  very  loose  make-up  in  the  interior.  The  connective  tissue  behind 
the  sphincter  thickens  in  the  fourth  year  of  life.  The  system  of  circular 
furrows  in  the  pigment  epithelium  I  have  first  seen  completely  developed 
in  a  7-year-old,  however. 

Yet  I  would  not  attach  too  much  weight  to  this  and  many  other  state- 
ments relating  to  age.     They  are  not  the  result  of  thorough  studies,  but 


EXTRAUTERINE  DEVELOPMENT  AND  GROWTH  OF  EYEBALL   209 

of  occasional  observations,  and  it  is  conceivable  that  a  great  variability 
rules  in  this  respect. 

The  development  of  the  definitive  form  of  the  ciliary  body  also  falls  in 
this  period.  This  depends  upon  the  further  carrying  out  of  processes 
which  have  already  begun  in  the  latter  period  of  fetal  life :  the  border  of 
the  retina  moves  farther  backward,  or,  more  correctly  stated,  the  meso- 
dermal layers  of  the  walls  of  the  eye  grow  farther  over  the  border  of  the 
retina.  The  material  for  the  covering  of  the  ever-broadening  orbiculus 
appears  to  be  that  in  the  transition  zone  between  the  retina  and  the 
ciliary  epithelium  resembling  the  embryonal  retina — that  which  is  so 
characteristic  of  the  eye  of  the  newborn.  In  a  2-year-old  child,  for 
example,  the  demarkation  of  the  retina  from  the  ciliary  epithelium  is 
sharp,  still  the  border  is  somewhat  rounded;  the  tendency  to  the  forma- 
tion of  the  fold  of  Lange  is  still  present.  In  the  7-year-old  child,  I  find 
the  same  relations  on  the  nasal  side  as  in  the  adult,  i.e.,  a  marked  pro- 
jection of  the  retina  over  the  ciliary  epithelium. 

The  complete  widening  out  of  the  angle  of  the  iris  appears  to  coincide 
with  the  backward  displacement  of  the  ciliary  processes  and  occurs  in 
the  period  between  the  second  and  fourth  years  of  life. 

In  general,  the  ciliary  processes  maintain  the  appearance  and  the 
dark  pigmentation  which  we  have  found  in  the  newborn  until  an  age 
of  later  childhood. 

According  to  Kerschbaumer  (117),  the  reticulum  of  H.  Mueller  is 
demonstrable  in  a  child  of  if  years;  still  it  is  then  very  delicate  in  any 
case.  In  sections  it  is  scarcely  apparent  at  this  age.  It  (as  well  as  the 
interlamellar  connective  tissue)  first  appears  in  later  childhood,  or  at  the 
time  of  puberty,  when  it  attains  its  complete  development. 

Unlike  these  processes  which  mostly  come  to  an  end  in  the  first 
part  of  chOdhood,  the  lens  grows  throughout  the  whole  of  life;  of  course, 
the  greatest  changes  in  it  are  found  in  the  first  year  of  life.  According  to 
Dub  (47),  the  ecjuatorial  diameter  is  7  .46  mm  at  the  age  of  10  to  11  months 
and  the  thickness  of  the  lens  2  .  46  mm.  The  lens,  therefore,  has  increased 
in  the  equatorial  direction  by  this  period,  and  has  also  become  thinner. 
The  explanation  of  this  lies  in  the  enlargement  of  the  anterior  segment, 
in  particular  of  the  ciliary  ring,  whereby  the  still  soft  and  plastic  lens  is 
displaced  in  the  frontal  direction — through  greater  tension. 

Both  diameters  increase  in  further  growth,  the  equatorial  to  a  greater 
extent  than  the  sagittal;  in  a  3-  to  35-year-old  child  the  lens  mass  is 
8.46X2.83  mm,  according  to  Dub.  This  further  change  of  the  lens-form 
apparently  has  its  ground  in  the  fact  that  the  new-built  fiber  layers  are 
thicker  in  the  equatorial  zone  that  at  the  poles,  for  this  fact  can  be  easily 


210  ANATOMY  AND  HISTOLOGY  OF  THE  HUMAN  EYEBALL 

established  anatomically.  For  example,  in  a  child  of  (probably)  1 2  years 
I  measured  the  thickness  of  the  less-stained  cortex  layers  at  the  equator  as 
0.34  mm,  at  the  anterior  pole,  however,  as  0.14  mm.  This  difference 
gradually  decreases  with  increasing  age,  and  the  thickness  of  the  new- 
formed  layers  becomes  more  uniform. 

An  estimate  of  the  growth  of  the  lens  is  rendered  difficult  by  the  fact  that  the 
various  figures  have  not  been  obtained  by  the  same  methods.  The  figures  of  von 
Pflugk  (172),  which  were  made  on  the  basis  of  an  eye  of  a  newborn,  were  obtained  in 
cross-section,  after  freezing,  the  figures  for  the  adult,  by  ophthalmometric  means; 
Dub  (47),  on  the  other  hand,  has  measured  isolated  lenses  and  estimated  their  thick- 
ness on  a  hard  base.  At  the  same  time  the  material  studied  is  much  too  small  to  rule 
out  disturbance  by  individual  differences. 

Now  we  know,  however,  that  the  severing  of  the  zonula  markedly  changes  the 
forms  of  the  lens,  and  this  change  is  still  greater  in  young  lenses  than  in  old;  indeed, 
in  general,  very  slight  forces  are  adequate  to  effect  a  notable  change  in  the  form  of  the 
lens  (Heine,  8q).  According  to  this,  the  measurements  contributed  are  not  comparable, 
necessarily. 

This  much  is  certain,  that,  aside  from  the  appositional  growth,  i.e.,  aside  from  the 
formation  of  new  lens  layers  on  the  surface,  still  other  factors  affect  the  form  of  the 
lens.  Such  a  factor  is  the  enlargement  of  the  ciliary  ring;  a  second  in  all  probability 
is  the  still-to-be-discussed  sclerosis  of  the  lens. 

The  inner  part  of  the  lens  possesses  a  greater  density,  even  in  child- 
hood; this  is  apparent  from  the  way  in  which  the  lens  substance  absorbs 
after  discission.  This  thickening  (sclerosis)  of  the  lens  substance  is,  there- 
fore, a  process  which  has  possibly  set  in  in  fetal  life  and  has  attained  such 
a  height  somewhere  in  the  thirties  that  spontaneous  absorption  is  impos- 
sible and,  therefore,  the  extraction  must  replace  the  discission. 

It  is  highly  probable  that  this  thickening  depends  upon  a  loss  of  water, 
and  that  this  goes  on  with  a  loss  of  volume.  That  such  a  shrinking 
must  in  part  compensate  for  the  increase  in  volume  as  a  result  of  the 
appositional  growth  is  easily  understood.  Yet  we  are  without  means  of 
stating  anything  accurately  concerning  the  degree  of  this  shrinking. 

The  older  the  person  is,  the  more  the  appositional  growth  of  the  lens 
decreases,  i.e.,  the  fewer  the  epithelial  cells  concerned  in  growth  at  any  one 
given  time.  The  influence  which  this  has  upon  the  appearance  of  the  lens 
vortex  has  already  been  spoken  of  (p.  169).  According  to  Becker  (18),  25 
cells  form  the  nuclear  bow  of  the  lens  in  the  newborn,  only  8  in  the 
4-year-old  child,  and  but  2  to  3  cells  in  the  older  person. 

When  the  whole  extrauterine  development  of  the  eyeball  is  surveyed,  one  sees 
at  once  that  a  few  processes  which  in  the  course  of  the  fetal  development  have  not 
been  completely  brought  to  closure,  e.g.,  the  development  of  the  fovea,  are  carried  on 
further. 

How  much  the  use  of  the  eye  as  a  sense  organ  has  an  influence  upon  its  develop- 


THE  APPEARANXES  OF  AGE  IN  THE  EYEBALL  211 

ment  still  remains  to  be  more  accurately  studied.  The  influence  of  light  upon  the 
development  of  the  medullary  sheaths  in  the  optic  nerve  appears  to  be  conclusi\-ely 
established.  Furthermore,  Grod  (77)  has  established  that  the  cornea  lags  behind 
o .  8  mm  on  the  average  in  its  growth  when  the  lens  is  removed  early  (at  the  age  of 
I  to  9  years).  Thereby  a  process  discovered  by  Wesseley  (236)  in  the  course  of 
animal  experiments  is  also  proven  true  for  man,  although  in  a  lesser  degree.  Of  course, 
the  question  still  remains  open,  whether  the  factor  which  causes  the  cornea  to  remain 
smaller  is  a  functional  insult  or  another,  possibly  a  mechanical  one. 

The  growth  of  the  eye  without  doubt  makes  greater  demands  upon  the  power 
of  resistance  of  the  tunics  of  the  bulb.  We  see  a  tendency  to  the  increase  and  thickening 
of  the  fibrillar  intervening  substance  of  the  connective  tissue,  at  first  of  the  collagenous, 
and  later  also  of  the  elastic  fibers.  According  to  Fuss  (71),  the  latter  attain  their  full 
development  at  the  age  of  10  to  11  years  and  increase  somewhat  in  number  from  there 
on  up  to  the  thirties.  The  thickening  of  the  cuticular  membranes  is  probably  to 
be  ascribed  to  the  same  cause. 

Finally,  it  is  to  be  remembered  that  the  optical  relations  also  change  decidedly  with 
the  growth  of  the  eye.  The  hjpermetropia  of  the  newborn  eye  is  not  sufficient  to 
compensate  for  the  later  addition  to  the  axial  length;  an  enlargement  of  the  main 
focalizing  limits  of  the  optical  system  must  occur,  therefore,  and  this  is  allected  for  the 
most  part  by  the  flattening  of  the  lens. 


CHAPTER  XIX.  THE  APPEARANCES  OF  AGE  IN  THE  EYEBALL 

There  are  changes  in  the  eyeball  which  gradually  develop  during  the 
whole  of  life  and  which,  therefore,  have  attained  a  higher  grade  in  old 
people  than  in  young.  To  these,  among  others,  belong  the  sclerosis  of  the 
lens  and  its  physiologic  equivalent,  the  decrease  of  the  accommodation, 
and  the  thickening  of  the  glass  membranes.  Other  changes  occur  first 
in  the  adult  eye,  but  during  the  period  of  complete  strength  and  vigor, 
and  increase  with  age,  like  the  cystoid  degeneration  of  the  retina.  None 
of  these  conditions  can  properly  be  characterized  as  senile;  to  a  certain 
extent  they  form  an  index  of  the  individual,  but  they  are  not  characteristic 
for  the  age  of  senility. 

Among  other  things,  the  arctis  senilis  of  the  cornea,  the  clouding  of  the 
lens,  the  so-called  verruca  of  the  chorioidea,  actually  do  set  in  in  senility 
or  a  few  years  earlier.  Therefore,  these  are  actual  senile  appearances. 
Many  of  these  can,  however,  also  appear  in  younger  individuals  as 
pathologic  processes,  and  possibly  they  are  often  such  in  senility.  In 
general,  therefore,  one  sees  how  uncertain  the  matter  is,  and,  in  particular, 
how  wide  the  play  for  subjective  conception. 

According  to  Priestley-Smith  (174),  the  diameter  of  the  cornea 
decreases  in  great  age;  the  average  from  40  to  60  years  is  11.48  mm, 
for  persons  over  60  years,  11.46  mm,  and  is  0.24  mm  smaller  than  the 


212  ANATOJNIY  AND  HISTOLOGY  OF  THE  HUMAN  EYEBALL 

average  from  20  to  30  years.  Prieslley-Smith  seems  to  think  of  a  true 
decrease  of  the  cornea;  it  is  also  possible,  however,  that  the  limbus 
becomes  more  clouded  and  so  limits  the  transparent  area. 

According  to  Steiger,  the  cornea  in  general  flattens  in  age.  More 
striking,  however,  is  the  frequency  of  perverse  astigmatism.  This  in- 
creases gradually  to  the  seventieth  year  of  life,  and  from  there  on  rapidly; 
the  vertical  meridian  of  the  cornea  flattens  more  than  the  horizontal. 

In  general,  the  sclera  increases  in  thickness  and  becomes  more 
rigid  and  less  distensible,  yet  this  thickening  does  not  go  so  far  that  a 
notable  limitation  of  the  interior  of  the  eyeball  arises. 

Throughout  the  whole  tunica  fibrosa  a  certain  degree  of  fatty  degenera- 
tion manifests  itself  in  age.  The  sclera  thereby  loses  its  porcelain- 
white  color  and  becomes  more  yellowish;  in  the  .cornea  the  degeneration 
makes  itself  manifest  by  clouding.  Yet  this  clouding  remains  confined 
to  the  marginal  portions  of  the  cornea  proper  (exclusive  of  the  limbus) 
and  is,  therefore,  called  arcits  senilis,  gerontoxon. 

According  to  Takayasu  (218),  the  fat  lies  in  the  intervening  sub- 
stance in  finest  round  or  elongated  drops;  the  degeneration  affects  first 
the  superficial  layers  of  the  corneal  stroma  and  extends  along  the  surface 
and  into  the  depth.  Little  fat  drops  also  appear  in  Bowman's  membrane, 
yet  these  are  finer  than  in  the  corneal  stroma.  Toward  the  periphery  the 
degeneration  is  superficially  limited  by  the  border  of  Bowman's  mem- 
brane; in  the  depths,  however,  it  progresses  farther  toward  the  sclera,  so 
that  the  peripheral  border  has  a  terraced  appearance  on  cross-section. 

In  the  uveal  tract  a  greater  development  of  the  collagenous  inter- 
mediary substance,  or  a  thickening  of  this,  makes  itself  manifest  before 
anything  else.  One  place  in  which  especially  dense  sclerotic  connective 
tissue  develops  is  the  anterior  half  of  the  bases  of  the  ciliary  processes. 
The  interstitial  connective  tissue  of  the  radial  portion  of  the  ciliary 
muscle  also  increases  in  amount,  and  the  muscle-bundles  thereby  become 
narrower.  The  interlamellar  connective  tissue  becomes  thicker  in  the 
same  manner. 

In  this  category  should  perhaps  be  placed  the  senile  miosis  and  es- 
pecially the  so-called  rigidity  of  the  sphincter  pupillae,  i.e.,  the  resistance 
of  the  pupil  to  dilatation.  It  supposedly  has  its  cause  in  the  thickening 
of  the  connective  tissue  behind  the  sphincter  (Fuchs,  67),  if  a  forma- 
tion of  hyalin  connective  tissue  at  the  border  of  the  pupil  is  not  the  cause, 
as  in  the  case  of  Meller  (149) ;  this  in  any  case  does  not  any  longer  belong 
in  the  territory  of  physiology. 

According  to  Kerschbaumer  (117),  changes  in  the  vessel  system  of  the 
chorioidea  are  frequent  even  in  the  fortieth  year  and  are  regular   in 


THE  APPEARANCES  OF  AGE  IN  THE  EYEBALL  213 

senility.  These  consist  especially  in  clouding  and  thickening  of  the 
vessel  walls,  loss  of  the  nuclei,  etc.  In  the  choriocapUlaris  this  condition 
expresses  itself  by  the  loss  of  the  endothelial  nuclei,  and  a  greater  accentua- 
tion of  the  vessel  contours;  the  whole  laj'er  thereby  becomes  more  stiff 
and  rigid. 

The  various  glands  and  membranes  (cuticular  membranes)  show  a 
further  and  greater  tendency  to  thickening  in  age  and  some  a  tendency 
to  localized  greater  thickening  or  wart  formation,  as  well. 

The  lens  capsule  only  increases  uniformly  in  thickness  (cf .  the  table  on 
p.  165);  the  Descemet's  membrane  gradually  thickens  in  the  center, 
but  warts  appear  more  and  more  plainly  at  the  border;  the  wart  zone 
broadens  (PI.  IV,  i).  Very  striking,  furthermore,  is  the  thickening  of 
the  limitans  interna  ciliaris,  especially  its  ridges.  The  results  of  this 
condition  upon  the  ciliary  epithelium  have  been  described  elsewhere 
(p.  122). 

Most  striking,  however,  is  the  tendency  to  the  formation  of  warts 
in  the  glass  membrane  of  the  chorioidea.  Since  this  appearance  may 
also  be  a  pathologic  process,  I  limit  myself  to  the  description  of  that 
form  which  is  found  in  the  eyes  of  old  people  without  other  pathologic 
changes. 

The  senile  warts  (vcrntcae  of  most  authors)  occur  at  the  periphery  of 
the  glass  membrane  of  the  chorioidea  as  small  to  medium  sized,  usually 
semispherical  elevations  (PI.  \TI,  7).  They  are  seated  immediately 
upon  the  glass  membrane,  so  that  one  can  follow  both  lamellae  of  the 
membrane  beneath  the  warts.  WTien  unstained,  the  warts  themselves 
(at  least  in  the  hardened  preparations)  consist  of  a  homogeneous,  glassy, 
transparent  substance;  this  stains  only  a  little  more  intensely  than  the 
cuticular  lamella.  Without  doubt  there  can  at  times  be  a  thin  stratum 
of  ordinary  cuticular  substance  on  the  surface  and  in  the  crevice  about 
the  base  of  the  small  warts;  this  substance  stains  feebly.  Since  the 
smallest  of  these  structures  also  set  directly  on  the  glass  membrane,  one 
must  look  upon  the  warts  as  a  secretion  product  of  the  pigment  epithelium. 
The  pigment  epithelium  courses  over  the  warts,  its  cells  are  larger  on  the 
summit  and  thinner,  and  the  nuclei  are  flattened. 

With  the  ophthalmoscope  the  warts,  therefore,  appear  somewhat 
lighter  than  their  environment,  and,  since  the  pigment  epithelium  to  a 
certain  extent  then  appears  in  profile  on  the  declivity  of  the  warts,  and 
a  thicker  layer  is  presented  here  (in  the  line  of  vision  of  the  observer), 
the  warts  appear  to  be  bordered  by  a  fine,  dark  seam.  The  peripheral 
situation  of  the  warts  makes  the  ophthalmoscopic  proof  of  them  more 
difficult,  because  they  are  to  be  seen  there  onl}'  in  the  less  magnified. 


214  ANATOMY  AND  HISTOLOGY  OF  THE  HUMAN  EYEBALL 

indirecl  image.  One  finds  them  decidedly  more  often,  therefore,  on 
anatomic  than  on  ophthahnoscopic  study. 

Variations  from  this  very  frequent  condition  occur  in  the  localization, 
the  grade  of  development,  and  the  structure  of  the  warts. 

As  long  as  the  formation  of  warts  involves  only  the  periphery  of  the 
fundus,  it  does  not  damage  the  function  of  the  eye.  At  times,  however, 
they  attack  the  territory  of  the  area  centralis  (and  very  extensively),  or 
they  attack  it  e.xclusively.  They  then  make  themselves  manifest  by  a 
lowering  of  the  visual  accuity  or,  indeed,  by  a  central  scotoma. 

Particularly  large  warts  imbed  themselves  deeper  in  the  retina, 
probably  also  lead  to  a  total  disappearance  of  the  pigment  epithelium 
over  the  summit,  and  thereby  to  further  secondary  changes  in  the 
retina.  Large  warts  at  times  show  a  stratification  about  a  core  which 
lies  at  the  base  of  the  wart  and  is  often  degenerated;  this  may  resemble 
particles  of  starch. 

A  calcification  is  frecjuently  found  in  these  warts,  as  also  in  those  in 
the  free  portions  of  the  glass  membrane  (H.  Mueller,  156;  Kerschbaumer, 
117).  One  then  finds  numerous,  very  fine,  highly  refractile  granules 
in  the  glass  membrane,  which  is  thereby  rendered  more  clouded  and 
fragile.  These  and  other  depositions  likewise  bring  out  a  clouding  in  the 
substance  of  the  warts.  When  the  overlying  pigment  has  disappeared, 
such  warts  appear  as  intensely  yellowish-white  flecks  or  show  a  plain, 
decided  reflex  (Nagel,  163),  especially  when  they  contain  larger  crystals. 

When  extensive  atrophic  processes  are  at  the  same  time  present  in  the  retina, 
when  the  capillaries  of  the  chorioidea  are  destroyed,  a  newly  built  connective  tissue  or 
bone  lies  beneath  the  warts,  and  one  has  a  pathologic  form  of  warts  to  deal  with.  Such 
changes  are  not  the  result  but  the  cause  of  the  wart  formation,  i.e.,  a  proliferation  of  the 
pigment  epithelium  and  an  abnormal  secretion  of  cuticular  substance.  The  stratified 
concrements  arising  primarily  in  the  cerebral  layer  of  the  retina  or  in  the  non- 
meduUated  section  of  the  optic  nerve  have,  in  general,  nothing  to  do  with  the  [pigment 
epithelium  and  are  pathologic  formations  throughout. 

The  pigment  epithelium  in  the  eye  of  the  aged  is  often  subject  to  a 
certain  amount  of  atrophy.  This  appears  partly  in  a  diffused  form,  and 
thereby  brings  out  the  tessellation  of  the  fundus  very  much  more  plainly, 
partly  in  a  circumscribed  form.  The  latter  occurs  in  the  immediate 
neighborhood  of  the  optic-nerve  entrance  and  leads  to  conus  or  halo-like 
figures.  ' 

Such  atrophic  conditions  are  often  found  in  the  pigment  epithelium 
in  the  neighborhood  of  the  border  of  the  retina  and  the  border  portions 
of  the  retina  are  frequently  fixed  thereby.  It  is,  however,  questionable 
whether  these  appearances  are  not  in  the  territory  of  the  pathologic. 


THE  APPEARANCES  OF  AGE  IN  THE  EYEBALL  215 

Concerning  the  development  of  cystoid  degeneration  there  has 
already  been  extended  discussion  (p.  84,  etc.). 

The  irregularities  appearing  in  the  ciliary  epithelium  and  their 
supposed  cause  have  also  been  referred  to  elsewhere.  However,  actual 
growths  of  the  ciliary  epithelium  may  also  come  about;  these  lead  to 
tufted  or  nodular  excrescences.  They  appear  on  the  crests  of  the  ciliary 
processes.  In  connection  with  the  more  marked  development  of  the 
connective  tissue  of  the  substratum,  they  bring  about  the  plump  appear- 
ance of  the  ciliary  processes  in  the  eye  of  the  aged,  especially,  however,  the 
nodular  and  racimose  appearance  of  the  ridges  (Kerschbaumer,  117;  Hess, 
loi),  and,  according  to  the  latter  author,  also  the  greater  extent  of  the 
whitish  coloration.  The  individual  excrescence  is  formed  by  the  ciliary 
epithelium  alone,  and  projects  over  the  surface  like  half  a  sphere  (PI. 
VH,  2). 

It  is  conceivable  that  not  only  the  ciliary  valleys  but  also  the  entire 
ciliary  ring  is  thereby  narrowed,  and  since,  too,  the  lens  is  at  the  same 
time  larger,  a  complete  closure  of  the  circumlental  space  can  come  about 
in  the  eye  of  the  aged. 

The  border  layer  of  the  vitreous  thickens  and  becomes  condensed 
in  age;  at  the  same  time  the  structure  of  the  nucleus  becomes  spaced  up, 
and  large  cavities  filled  with  fluid  appear  (liquefaction  of  the  vitreous, 
synchisis).     The  zonula  fibers  are  also  greatly  thickened  in  age. 

The  lens  increases  in  size  up  to  the  very  greatest  age,  and  although 
the  rapidity  of  its  growth  is  very  much  less  than  that  in  youth,  a  much 
longer  time  is  allowed  for  it.  Accurate  statements  concerning  the  growth 
of  the  equatorial  diameter  have  been  made  by  Priestley-Smith  (173). 
In  the  third  decade  of  life  it  has  an  average  of  8 .  67  mm,  in  the  eighth 
decade  9.64  mm.  Along  with  this  there  is  always  a  further  individual 
play  of  at  least  0.75  mm. 

According  to  the  same  author,  such  senile  lenses  may  be  5  to  6  mm 
or  more  thick.  The  ophthalmometric  measurements  of  Saunte  (pub- 
lished by  Tscherning,  2 28)  present  a  continuous  increase  of  the  thickness 
of  the  lens  from  3 . 6  mm,  in  persons  under  20,  up  to  4 . 5  mm  in  persons 
over  50  years  of  age. 

From  all  this  it  is  evident  that  at  this  period  of  life  the  lens  increases 
uniformly  in  all  diameters.  According  to  Priestley-Smith,  one  can, 
therefore,  deduce  the  form  of  the  senile  lens  from  that  of  the  youthful 
lens  if  one  thinks  of  it  as  surrounded  by  a  shell  some  o .  5  mm  thick.  The 
margin  of  the  senile  lens  is,  therefore,  much  more  strongly  rounded. 

The  yellow  coloring  of  the  lens  often  attains  such  a  height  in  great 


2i6  ANATOMY  AND  HISTOLOGY  OF  THE  HUMAN  EYEBALL 

age  that  the  color  perception,  especially  that  for  blue,  becomes  markedly 
disturbed  (Hess,  98). 

Corresponding  to  the  slight  growth  of  the  lens  fibers  in  age,  the  nuclear 
bow  of  the  lens  shows  only  a  few  transitional  divisions  from  epithelium 
to  lens  substance,  and  the  nuclear  bow  bends  sharply  and  often  very 
irregularly  forward  (PI.  IX,  3). 

It  is  v^ery  difficult  to  decide  to  what  extent  opacifications  should  be  accredited  as 
senile  appearances.  It  is,  indeed,  true  that  one  practically  never  sees  an  eye  in  the 
aged  in  which  the  lens  is  completely  free  from  clouding.  Yet  this  clouding  does  not 
progress  in  all  people  to  a  complete  formation  of  cataract.  The  senile  cataract  and  all 
its  initial  stages,  frequent  as  they  are,  had,  therefore,  better  be  treated  as  pathologic 
appearances. 

One  result  of  the  enlargement  of  the  lens,  in  part  also  of  the  defective 
secretion  of  aqueous,  is  the  lessened  depth  of  the  anterior  chamber.  That 
the  free  space  of  the  posterior  chamber  is  also  narrowed,  and  in  a  still 
higher  degree,  is  shown  by  the  enlargement  and  thickening  of  the  ciliary 
processes,  the  lens,  and  the  zonula  fibers. 


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klinische  Wochenschr.  1888,  Nr.  11. 

(36)  Einiges  zur  Lchre  von  der  Enlstehung  und  dem  Verlaufe  des  prodromalen 

und  akuten  Glaukomanfalles.     Prager  medizinische  Wochenschr.  1897. 

(37)  Dedekind,  Beitriigc  zur  Entwicklungsgcschichtc  der  Augcngcfasse  des  Menschen. 
Anatomische  Hefte,  Bd.  38,  pag.  i  (1909). 

(38)  Deyl,  Uber  den  Eintritt  der  Arteria  centralis  retinae  in  den  Sehnerven  beim 
Menschen.     Anatom.  Anzciger,  Bd.  11,  pag.  687  (1896). 

(39)  Dimmer,  Die  ophthalmoskopischen  Lichtrefiexc  der  Netzhaut.  Leipzig  und  Wien 
1891. 

(40)  Beitrage  zur  .■\natomic  und  Physiologic  der  Macula  lutea  des  Menschen. 

Leipzig  und  Wien    1894. 

(41)  Demonstration    von    Photogrammen    nach    Schnittpraparaten    durch    die 

Fovea.     Bcricht  u.  d.  30.  Versamml.  d.  ophthalm.  Gcscllsch.  in  Heidelberg  1902,  pag.  362. 

(42)  Die  Macula  lutea  der  menschlichcn  Netzhaut  und  die  durch  sic  bedingten 

entoptischcn  Erschcinungen.     v.  Graefes  Archiv,  Bd.  65,  pag.  486  (1907). 

(43)  DoGlEL,  Die  Ncrven  dcr  Cornea  des  Menschen.  Anatom.  Anzciger,  Bd.  5,  pag.  483 
(1890). 

Die  Nervenendkorperchen  (Endkolbcn,  W.  Krausc)  in  dcr  Cornea  und  Conjunctiva 

bulbi  des  Menschen.     Archiv  f.  mikroskop.  Anatomic,  Bd.  37,  pag.  602  (1891). 

(44)  Zur  Fragc  iiber  den  Bau  der  NervenzcUcn.     Archiv  f.  mikroskop.  Anatomic, 

Bd.  41,  pag.  62  (1893). 

(45)  DoNDERS,  Die  Grenzen  des  Gesichtsfeldes  in  Bczichung  zu  dcncn  dcr  Netzhaut. 
v.  Graefes  Archiv,  Bd.  23,  Abt.  2,  pag.  255  (1877). 

(46)  Drualt,  Appareil  de  la  vision,  in  Poirier  et  Charpy,  Traite  d'anatomie  humaine, 
Tome  V. 

(47)  Dub,  Beitrag  zur  Kenntnis  der  Cataracta  zonularis.  v.  Graefes  Archiv,  Bd.  37, 
Abt.  4,  pag.  26  (1891). 

(48)  v.  Ebner,  Untersuchungen  iiber  die  Ursachcn  dcr  Anisotropic  organisicrter  Sub- 
stanzen.    Leipzig  1882 — nach  H.  Viechow  (233). 


LITERATURE  CITATIONS  219 

(49)  Elschnig,  Optico-ciliares  Gefass.     Archiv  f.  Augenheilk,  Bd.  18,  pag.  295  (1888). 

(50)  Cilio-retinale  Gefasse.    v.  Graefes  Archiv,  Bd.  44,  Abt.  i,  pag.  144  (1897). 

(51)  Uber  optico-ciliare  Gefasse.     Klin.  Monatsbl.  f.  Augenheilk.,  36.  Jahrg., 

pag.  93  (1898). 

(52)  Der    normale    Sehnerveneintritt    des    menschlichen    Auges.    Denkschr.    d. 

mathem.-naturw.  Klasse  d.  k.  Akademie  d.  Wissensch.  in  Wien,  Bd.  70  (1900). 

(53)  Histologische  Artefakte  im  Sehnerven.    Klin.  Monatsbl.  f.  Augenheilk.,  40. 

Jahrg.,  Bd.  II,  pag.  81  (1902). 

(54)  Bemerkungen  iiber  die  Refraktion  der  Neugeborenen.  Zietschr.  f.  Augen- 
heilk.,  Bd.    II,  pag.    10   (1905). 

(55)  Elschnig  untj  Lauber,  Uber  die  sogenannten  Klumpenzellen  der  Iris.  v.  Graefes 
Archiv,  Bd.  65,  pag.  428  (1907). 

(56)  Elze,  Beschreibung  eines  menschlichen  Embrj-o  von  zirka  7  mm  grosster  Lange. 
Anatom.  Hefte,  Bd.  35,  pag.  409  (1908). 

(57)  Embden,  Primitivfibrillenverlauf  in  der  Netzhaut.  Archiv  f.  mikroskop.  Anatomie, 
Bd.  57  (1901). 

(58)  EwiNG,  Uber  ein  Bauverhaltnis  des  Irisumfanges  beim  Menschen.  v.  Graefes  Archiv, 
Bd.  34,  Abt.  3,  pag.  i  (1888). 

(59)  FoRSMARK,  Zur  Kenntnis  der  Irismuskulatur  des  Menschen;  ihr  Bau  und  ihre 
Entwicklung.  Mitteilungen  aus  der  Augenklinik  d.  Carol,  medico-chirurg.  Inst,  in  Stockholm, 
Heft  7  {1905). 

(60)  Freytag,  Uber  von  Einfluss  der  Linsenveranderungen  auf  die  Refraktion  des 
Auges.    Archiv.  f.  Augenheilk.,  Bd.  54,  pag.  328  (1906). 

(61)  \'ergleichende  Untersuchungen  iiber  die  Brechungsindices  der  Linse  und  der 

flussigen  Augenmedien  des  Menschen  und  der  hoheren  Tiere  in  verschiedenen  Lebensaltem. 
Wiesbaden  1907. 

(62)  Friden"berg,  Uber  die  Figur  des  Linsensternes  beim  Menschen  und  einigen  Wirbel- 
tieren.    Archiv  f.  Augenheilk.,  Bd.  31,  pag.  293  (1895). 

(63)  Fritsch,  \'ergleichende  Untersuchungen  menschlicher  Augen.  Sitzungsber.  d.  kgl. 
preusz.  Akademie  d.  Wissensch.,  Bd.  30,  pag.  636 — Anatom.  Anzeiger,  Bd.  30,  pag.  462  (1907) 
— X'erhandlungen  d.  anatom.  Gesellsch.,  22.  Versamml.,  pag.  141  (1908). 

(64)  Fritz,  Uber  den  \'erlauf  der  Nen-en  im  vorderen  Augenabschnitte.  Sitzungsber. 
d.  mathem.-naturw.  Klasse  d.  k.  Akademie  d.  Wissensch.  in  Wien,  Bd.  113,  pag.  273. 

(65)  FucHS,  E.,  Beitrage  zur  normalen  Anatomie  des  Augapfels.  v.  Graefes  Archiv, 
Bd.  30,  Abt.  4,  pag,  i  (1884). 

(66)  Die  periphere  Atrophic  des  Sehnerven.    v.  Graefes  Archiv,  Bd.  31,  Abt.  i, 

pag.  177  (1885). 

(67)  Beitrage  zur  normalen  Anatomie  der  menschlichen  Iris.     v.  Graefes  Archiv, 

Bd.  31,  Abt.  3,  pag.  39  (1885). 

(68)  Uber  Komplikationen  der  Heterochromie.     Zeitschr.  f.  Augenheilk.,  Bd.  15, 

pag.  191  (1906). 

(69)  Lehrbuch  der  AugenheiUcunde.     12.  Aufl.    Wien  und  Leipzig  1910. 

(70)  FuxALA,  Was  ist  die  Aufgabe  des  Briickeschen  Muskels?  Archiv  f.  Augenheilk., 
Bd.  36,  pag.  6s  (1898). 

(71)  Fusz,  Zur  Frage  des  elastischen  Gewebes  im  normalen  und  myopischen  Auge. 
Virchows  Archiv,  Bd.  183,  pag.  465  (1906). 

(72)  Garnier,  Uber  den  normalen  und  pathologischen  Zustand  der  Zonula  Zinnii. 
Archiv  f.  Augenheilk.,  Bd.  24,  pag.  32  (1891). 

(73)  Garten,  Die  \'eranderungen  der  Netzhaut  durch  Licht,  in  Graefe-Saemisch, 
Handbuch  d.  ges.  Augenheilk.,  2.  Aufl.,  I.  Teil,  Kap.  XII,  Anhang  pag.  72  (1908). 

(74)  Greeff,  Das  Wesen  der  sogenannten  Fuchsschen  Atrophic  des  Sehnerven.  IX. 
internal,  ophthalm.  Kongress,  Utrecht  1899,  pag.  87. 


220  ANATOMY  AND  HISTOLOGY  OF  THEHUMAN  EYEBALL 

(75)  Mikroskopische   Anatomic    des  Sehncrven  und  dcr  Netzhaut,  in  Graefe- 

Saemisch,  Handbuch  d.  ges.  Augenheilk.,  2.  Aufl.,  I.  Teil.  Bd.  I,  Kap.  V  (igoo). 

(76)  Uber  cine  Fovea  externa  in  dcr  Retina  dcs  Menschen.    Bcrichl  u.  d.  30. 

Versamml.  d.  opthalm.  Gesellsch.  in  Heidelberg  1902,  pag.  160. 

(77)  Grod,  Uber  die  Dauerresultate  dcr  Operationen  bei  angeborcncni  Star.  Arch.  f. 
Augenheilk.,  Bd.  67,  pag.  251  (1910). 

(78)  Groenouw,  Intrasklcrale  Nervenschleifen.  KJin.  Monatsbl.  f.  Augenheilk.,  43. 
Jahrg.,  Bd.  I,  pag.  637  (1905). 

(79)  Grunert,  Der  Dilatator  pupillae  des  Menschen,  ein  Beitrag  zur  Anatomic  und 
Physiologic  der  Irismuskulatur.    Archiv  f.  Augenheilk.,  Bd.  36,  pag.  319  (1898). 

(80)  Grynfeltt,  Le  muscle  dUatateur  dc  la  pupOle.    Montpellier  1899. 

(81)  GuLLSTRAND,  Die  Farbe  der  Macula  centralis  retinae,  v.  Gracfcs  Archiv,  Bd.  62, 
pag.  I  (1905). 

(82)  GuTMANN,  Zur  Histologic  der  Ciliarner\-en.  Archiv  f.  mikroskop.  Anatomic,  Bd.  49, 
pag.  I  (1897). 

(83)  Uber  koUagenes  und  protoplasmatisches   Gewebe  der  menschlichen  Iris. 

Zeitschr.  f.  Augenheilk.,  Bd.  10,  pag.  8  (1903). 

(84)  Hahn,  Untersuchungcn  iiber  den  histologischen  Bau  der  Ciliamerven.  Wiener 
klinische  Wochenschr.  1897,  pag.  714. 

(85)  Halben,  Ein  Differentialrefraktometer  zur  Bestimmung  dcr  Brechungsindices 
optisch  inhomogener  Medien,  spcziell  der  menschlichen  Linse.  Bcricht  ii.  d.  32.  Versamml. 
d.  ophthalm.  Gesellsch.  in  Heidelberg  1905,  pag.  354. 

(86)  Hamburger,  Besteht  freie  Kommunikation  zwischen  vorderer  und  hinterer  Augen- 
kammer?    Zentralbl.  f.  pr.  Augenheilk.,  22.  Jahrg.,  pag.  225  (1898). 

(87)  Hannover,  Funiculus  scleroticae,  un  restc  dc  la  fcnte  foetalc  dans  I'oeil  humain. 
Kopenhagcn  1876 — Nagels  Jahresbericht  f.  1876. 

(88)  Heerfordt,  Studien  iiber  den  Musculus  dilatator  pupillae.  Anatom.  Hefte,  Bd.  14, 
pag.  487  (1900). 

(89)  Heine,  Beitrage  zur  Physiologic  und  Pathologic  der  Linse.  v.  Graefes  Archiv, 
Bd.  46,  pag.  52s  (1898). 

(90)  Die  Anatomic  des  akkommodierten  Augcs.     v.  Gracfcs  Archiv,  Bd.  49,  pag.  i 

(1899)- 

(91)  Beitrage  zur  Anatomic  des  myopischen  Auges.    Archiv.  f  Augenheilk.,  Bd.  38, 

pag.  277  (1899). 

(92)  Demonstration  des  Zapfenmosaiks  der  menschlichen  Fovea.     Bcricht  ii.  d. 

29.  Versamml.  d.  ophthalm.  Gesellsch.  in  Heidelberg  1901,  pag.  265. 

(93)  Held,  Zur  weiteren  Kenntnis  der  Nervendfiisse  und  zur  Struktur  der  Sehzcllen. 
Abhandlg.  d.  mathem.-physikal.  Klasse  d.  k.  sachs.  Gesellsch.  d.  Wissensch.,  Nr.  2,  pag.  145 
— Nagels  Jahresbericht  f.  1904,  pag.  23  (1904). 

(94)  Hexle,  Eingeweidelehre,  Braunschweig  1866. 

(95)  Herzog,  tfber  die  Entwicklung  der  Binnenmuskulatur  dcs  Augcs.  Archiv  f. 
mikroskop.  Anatomic,  Bd.  60  (1902). 

(96)  Hess,  Arbeiten  aus  dcm  Gcbeite  dcr  Akkommodationslehre.  v.  Graefes  Archiv, 
Bd.  42,  Heft  I,  pag.  288  (1895). 

(97)  Pathologie  und  Therapic  des  Linscnsystems  in  Graefe-Saemisch,  Handbuch 

d.  ges.  Augenheilk.,  2.  Aufl.,  II.  Teil,  Bd.  VI,  2.  Abtcil.,  Kap.  IX  (1905). 

(98)  tjber  Blaublindheit  durch  Gelbfarbung  der  Linse.     Archiv  f.  Augenheilk., 

Bd.  61,  pag.  29  (1908). 

(99)  Die  Akkommodation  bei  Tauchervogeln.    Archiv  f.  vergleichende  Ophthalm., 

Bd.  I,  pag.  153  (1910). 

(100)  Hess,  Beitrage  zur  Kenntnis  akkommodativer  Anderungen  im  menschlichen  Auge. 
V.  Graefes  Archiv,  Bd.  65,  pag.  170  (1910). 


LITERATURE  CITATIONS  221 

(loi)  Uber  individuelle  \'erschiedenheiten  des  normalen  Ciliarkorpers.     Archiv  f. 

Augenheilk.,  Bd.  67,  pag.  341   (1910). 

(102)  V.  HiPPEL,  E.,  Uber  das  normale  Auge  des  Xeugeborenen.  v.  Graefes  Archiv, 
Bd.  45.  pag.  2S6  (1898). 

(103)  HiRSCH,  1st  die  fotale  Hornhaut  vaskularisiert  ?  Klin.  Monatsbl.  f.  .Augenheilk., 
44.  Jahrg.,  Bd.  II,  pag.  13  (1906). 

(104)  HlRSCHBERG,  Beitriige  zur  .Vnatomie  und  Pathologie  des  Auges.  Archiv  f.  Augen- 
heilk., Bd.  8,  pag.  49  (1879). 

(105)  His,  Abbildungen  uber  das  Gefiiss-System  der  menschlichen  Netzhaut  und  der- 
jenigen  des  Kaninchens.    Archiv  f.  Anatomie  u.  Physiologie,  anatom.  Abteil.  1880,  pag.  224. 

(106)  HosCH,  Das  Epithel  der  vorderen  Linsenkapsel.  v.  Graefes  Archiv,  Bd.  20,  Heft  i, 
pag.  83  (1874)  und  Bd.  52,  pag.  484  (1901). 

(107)  HoYER,  tJber  die  Nerven  der  Hornhaut.  Archiv  f.  mikroskopische  Anatomie, 
Bd.  9,  pag.  220  (1873). 

(108)  V.  Jaeger,  E.,  Uber  die  Einstellungen  des  dioptrischen  Apparates  im  mensch- 
lichen Auge.    Wien  1861 — nach  Weiss  (235). 

(109)  Jakoby,  Uber  die  Neuroglia  des  Sehner\-en.  Klin.  Monatsbl.  f.  Augenheilk., 
43.  Jahrg.,  Bd.  I,  pag.  129  (1905). 

(no)  Jeannulatos,  Recherches  embrj'ologiques  sur  le  mode  de  formation  de  la  chambre 
anterieure  chez  les  mammiferes  et  I'homme.    These  de  Paris  1896 — nach  Seefelder  (203). 

(in)  IscHREYT,  Uber  den  Faserbiindelverlauf  in  der  Lederhaut  des  Menschen.  v.  Graefes 
Archiv,  Bd.  48,  pag.  506  (1899). 

(112)  JuSELius,  Die  Entwicklung  des  hinteren  Pigmentepithels  der  Iris  aus  der  sekun- 
daren  Augenblase  und  sein  Verhaltnis  zur  Irismuskulatur  und  den  spontanen  Iriszysten. 
Klin.  Monatsbl.  f.  Augenheilk.,  46.  Jahrg.,  Bd.  II,  pag.  19  (1908). 

(113)  IwANOFF,  Beitrage  zur  normalen  und  pathologischen  Anatomie  des  Auges. 
v.  Graefes  Archiv,  Bd.  11,  Abt.  i,  pag.  135  (1865). 

(114)  Beitrage  zur  Anatomie  des  Ciliarmuskels.     v.  Graefes  Archiv,  Bd.  15,  Abt.  3, 

pag.  284  (1869). 

(lis)  Der  Uvealtraktus   in   Graefe-Saemisch,   Handbuch  d.  ges.  Augenheilk., 

I  Aufl.,  Bd.  I,  Kap.  Ill  (1874). 

(116)  Keibel,  Normentafeln  zur  Entwicklungsgeschichte  des  Menschen,  Heft  8.  Jena 
1908. 

(117)  Kerschb.\umer,  R.,  Uber  Altersveranderungen  der  Uvea.  v.  Graefes  Archiv, 
Bd.  34,  Abt.  4,  pag.  16  (1888)  und  Bd.  38,  Abt.  1,  pag.  127  (1892). 

(118)  KiRiBCCHi,  Uber  die  Fuchssche  periphere  Atrophic  des  Sehnerven.  Archiv  f. 
Augenheilk.,  Bd.  39,  pag.  76  (1899). 

(119)  KoGANi;!,  Untersuchungen  iiber  den  Bau  der  Iris  des  Menschen  und  der  Wirbel- 
tiere.    Archiv  f.  mikroskop.  .-Vnatomie,  Bd.  25,  pag.  i  (1885). 

(120)  Kolliker,  Handbuch  der  Gewebelehre  des  Menschen.  Bd.  3,  bearbeitet  von 
V.  Ebker,  1902. 

(121)  Koxigstein,  Histologische  Notizen,  IV.  Das  Wachstum  des  embr>'onalen  .Auges. 
V.  Graefes  Archiv,  Bd.  30,  Abt.  i,  pag.  135  (1884). 

(122)  Krause,  Die  Nerven  der  Arteria  centralis  retinae,  v.  Graefes  Archiv,  Bd.  21, 
Abt.  I,  pag.  296  (187s). 

(123)  Krijckmann,  Uber  Pigmentierung  und  Wucherung  der  Netzhautneuroglia. 
v.  Graefes  Archiv,  Bd.  60,  pag.  350  und  452  (1905). 

(124)  Uber  die  Entwicklung  und  Ausbildung  der  Stiitsubstanz  im  Sehnerven  und 

der  Netzhaut.     Klin.  Monatsbl.  f.  .\ugenheilk.,  44.  Jahrg.,  Bd.  I,  pag.  162  (1906). 

(125)  Erkrankungen  der  Uvea,   in  A.xexfeld,  Lehrbuch   der  Augenheilkunde. 

Jena  1909. 

(126)  KtJHNE,  in  Hermanns  Handbuch  der  Physiologie,  Bd.  3. 


222  ANATOMY  AND  HISTOLOGY  OF  THE  HUMAN  EYEBALL 

(127)  Kuiwr,  Zur  Kenntnis  dcs  Pigmenlcpithcls.  Zcntralblatt  f.  d.  mcdizin.  Wissenscb. 
1877,  pag.  337. 

(128)  Zur  Architcktonik  der  Retina.     Bericht  ii.  d.  10.  \'ersaniml.  d.  ophthalm. 

Gesellsch.  in  Heidelberg  1877  in  klin.  Monatsbl.  f.  Augenheilk.,  15.  Jahrg.,  Beilageheft  pag.  72. 

(129)  Zur  Kenntnis  des  Sehnerven  und  der  Netzhaut.    v.  Graefes  Archiv.  Bd.  25, 

Abt.  3,  pag.  179  (1897). 

(130)  Uber  den  Bau  der  Fovea  centralis  des  Menschen.     Bericht  ii.  d.  13.  Ver- 

samml.  d.  ophthalm.  Gesellsch.  in  Heidelberg  1881,  in  klin.  Monatsbl.  f.  Augenheilk.,  19.  Jahrg., 
Beilageheft  pag.  141. 

(131)  Uber  einige  Allersveranderungen  im  menschlichen  Auge.     Ibidem  pag.  38. 

(132)  KusCHEL,  Die  Architektur  dcs  Auges  und  ihre  hydrostatischen  Beziehungen  zum 
intraokularen  Stromgefalle.     Zeitschr.  f.  Augenheilk.,  Bd.  17,  pag.  114  (1907). 

(133)  Lang  and  Barrett,  On  the  frequency  of  cilio-retinal  vessels.  Ophthalm.  Hosp. 
Reports,  Vol.  12,  p.  59  (1888). 

(134)  Lange,  Vorzeigung  infantiler  Netzhautpriiparate.  Bericht  ii.  d.  23.  Versamml.  d. 
ophthalm.  Gesellsch.  in  Heidelberg  1893,  pag.  236. 

(135)  Zur  Anatomic  des  Auges  des  Neugeborenen.     Klin.  Monatsbl.  f.  Augenheilk., 

39.  Jahrg.,  pag.  i,  202  (1901). 

(136)  Langer,  1st  man  berechtigt,  den  Perichorioidalraum  und  den  Tenonschen  Raum 
als  Lymphraume  aufzufassen?  Sitsungsber.  d.  mathem.-naturw.  Klasse  der  k.  Akad.  d. 
Wissensch.  in  Wien,  Oktober  1890,  Bd.  99,  Heft  3. 

(137)  Lauber,  Beitrage  zur  Entwicklungsgeschichte  und  Anatomie  der  Iris  und  des 
Pjgmentepithels  der  Netzhaut.    v.  Graefes  Archiv,  Bd.  68,  pag.  i  (1908). 

(138)  Leber,  Th.,  Die  Zirkulations-  und  Emahrungsverhiiltnisse  des  Auges,  in  Graefe. 
Saemisch,  Handbuch  der  ges.  Augenheilk.,  2.  Aufl.,  I.  Teil,  Bd.  2,  XI.  Kap.  (1903). 

(139)  Leboucq,  Contribution  a  I'etude  de  la  retine  chez  les  mammiferes.  Arch.  dAna- 
tomie  microscop.  1909,  pag.  155 — Nagels  Jahresbericht  f.  1909,  pag.  7. 

(140)  V.  Lenhossek,  Die  Entwicklung  des  Glaskorpers.     Leipzig  1903. 

(141)  DE  LlETO  Vollaro,  Sulla  esistenza  nella  cornea  di  fibre  elastiche  colorabili  co! 
metodo  di  Weigert.     Loro  derivazione  dai  corposcoh  fissi.     Annali  di  Ottahn.     1907,  pag.  713. 

(142)  Sulla    morfologia   della    membrana   dilatatrice   deUa   pupilla   dell'    uomo 

Annali  di  Ottalm.,  Bd.  37,  pag.  301  (1908)— Nagels  Jahresber.  f.  1908,  pag.  16. 

(143)  II  tessuto  elastico  nell'  iride  dell'  uomo  adulto  e  di  alcune  specie  di  vertebrati. 

•J  AiqDJV  vergl.  Ophthalm.,  i.  Jahrg.,  pag.  49  (1910)- 

(144)  LOHMANN,    Uber   die    typische   Exzentrizitiit    des   kieinen  Irisringes Klin. 

Monatsbl.  f.  Augenheilk.,  44.  Jahrg.,  Bd.  II,  pag.  68  (1906). 

(145)  Lowenstein,  Uber  regionare  Anasthesie  in  der  Orbita.  Kim.  Monatsbl.  f.  Augen- 
heilk., 46.  Jahrg.,  Bd.  I.,  pag.  592  (1908). 

(146)  Magnus,  Uber  Blasenbildung  am  Linsenaquator.  Klin.  Monatsbl.  f.  Augenheilk., 
29.  Jahrg.,  pag.  291  (1891). 

(147)  Marx,  Die  Ursache  der  roten  Farbe  des  normalen  ophthalmoskopisch  beobachteten 
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(148)  Matthiessen,  Uber  das  Gesetz  der  Zunahme  der  Brechungsindices  innerhalb 
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(149)  Meller,  Uber  hyaline  Degeneration  des  Pupillarrandes.  v.  Graefes  Archiv,  Bd.  59, 
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(i  50)  Die  histologischen  Veranderungen  des  Auges  bei  Keratitis  disciformis.     Klin. 

Monatsbl.  f.  Augenheilk.,  43.  Jahrg.,  Bd.  II,  pag.  335  (1905)- 

(151)  Merkel  und  Kallius,  Mikroskopische  Anatomie  des  Auges,  in  Graefe-Saemisch, 
Handbuch  d.  ges.  Augenheilk.,  2.  Aufl.,  I.  Teil,  Bd.  I,  Kap.  I  (1901). 

(152)  Merkel  und  Orr,  Das  Auge  des  Neugeborenen  an  einem  schematischen  Durch- 
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LITERATURE  CITATIONS 


223 


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Bd.  17,  pag.  324  (1879). 

(154)  MlCHAELls,  Altersbestimmung  menschlicher  Embn-onen.  Archiv  f.  G\iiakologie, 
Bd.  78,  pag.  267  (1906). 

(155)  Michel,  tTaer  Iris  und  Iritis,    v.  Graefes  Archiv,  Bd.  27,  Abt.  2,  pag.  171  (1881). 

(156)  MuLLER,  Heixr.,  Anatomische  Beitrage  zur  Ophthalmologic,  v.  Graefes  Archiv, 
Bd.  2,  Abt.  2,  pag.  I  (1855). 

(157)  Anatomische  Beitrage  zur  Ophthalmologie.    v.  Graefes  Archiv,  Bd.  3,  pag.  i 

(1857). 

(158)  Anatomisch  -physiologische  Untersuchungen  tiber  die  Retina  des  Menschen 

und  der  Wirbelticre.    Zeitschr.  f.  wissensch.  Zoologic,  Bd.  8,  pag.  i  (1857) — Greeff  (75). 

(159)  tjber  glatte  Muskelfasem  und  Nervengcflechte  der  Chorioidca  im  mensch- 

lichen  Auge.     Verhandl.  d.  Wiirzburg.  physik.-mediz.  Gesellsch.,  Bd.  X,  Heft  2  und  3,  pag. 
179. — Gesammelte  Schriften,  pag.  201. 

(160)  MuxCH,  Uber  die  muskulose  Natur  des  Stromazellnetzes  der  Uvea.  Zeitschr.  f. 
Augenheilk.,  Bd.  12,  pag.  525  (1904). 

(161)  Zur  Anatomic  des  Dilatator  pupillae.     Zeitschr.  f.  Augenheilk.,  Bd.   13, 

pag.   I    (1905). 

(162)  — • Uber  die  Innervation  der  stromazellen  der  Iris.     Zeitschr.  f.  Augenheilk., 

Bd.  14.  pag.  130  (1905). 

(163)  Nagel,  Hochgradige  Amblyopic,  bedingt  durch  glashautige  \\'ucherungen  and 
kristaUinische  Kalkablagerungen  an  der  Innenflache  der  Aderhaut.  Klin.  Monatsbl.  f. 
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(164)  Naito,  Ein  Beitrag  zur  Kenntnis  der  intraskleralen  Nervenschleifcn.  Klin. 
Monatsbl.  f.  Augenheilk.,  40.  Jahrg.,  Bd.  II,  pag.  122  (1902). 

(165)  Nettleship,  Cilio-retinal  blood-vessels.  Ophthalm.  Hosp.  Rep.,  Vol.  8,  p.  512  and 
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(166)  Nuel,  Dc  la  vascularisation  dc  la  choroide  ct  de  la  nutrition  de  la  retine  prin- 
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(167)  Oeller,  Atlas  der  Ophthalmoskopie  1897,  A,  Taf.  IV,  3.  Lieferung. 

(168)  Passera,  La  rete  vascolare  sanguigna  dcUa  membrana  coriocapillare  dell'  uomo. 
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(169)  Pause,  Uber  die  Nerven  der  Iris.    v.  Graefes  Archiv,  Bd.  23,  Abt.  3,  pag.  i  (1877). 

(170)  Pes,  Uber  cinige  Besondcrheiten  in  der  Struktur  der  mcnschlichen  Cornea.  Archiv 
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(171)  — ■ SuUa  fina  anatomia  della  sclerotica.     Annali  di  Ottalm.,  Bd.  37,  pag.  331  — 

Nagels  Jahresbcricht   f.    1908. 

(172)  v.  Pflugk,  Die  Fixierung  der  Wirbcltierlinsen,  insbesondcre  der  Linse  des  neu- 
geborenen  Menschen.     Klin.     Monatsbl.  f.  Augenheilk.,  47.  Jahrg.,  Bd.  II,  pag.  i  (1909). 

(173)  Priestley-Sihth,  On  the  growth  of  the  crystalline  lens.  Transactions  of  the 
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(174)  On  the  size  of  the  cornea  in  relation  to  age,  sex,  refraction  and  primary 

glaucoma.    Transactions  of  the  Ophth.  Soc.  of  the  U.  K.,  Vol.  10,  p.  68  (1891). 

(175)  Rabl,  tJbcr  den  Bau  und  die  Entwicklung  der  Linse  (3.  Teil),  Zeitschr.  f.  wissensch. 
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(176)  Raehlmann,  Zur  Anatomic  und  Physiologic  des  Pigmentepithels  der  Netzhaut. 
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(177)  V.  Recklinghausen,  Die  Lymphgefasse  und  ihre  Beziehung  zum  Bindegewebc. 
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(178)  Reimar,  Uber  die  ophthalmoskopische  Sichtbarkcit  der  Ora  serrata  und  der  Pro- 
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(179)  Retzius,  Zur  Kenntnis  vom  Bau  der  Iris.  Biologische  Untersuchungen,  N.F., 
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224  ANATOMY  AND  HISTOLOGY  OF  THE  HUxMAN  EYEBALL 

(iSo)  Uber  den  Bau  des  (ilaskorpcrs  und  der  Zonula  Zinnii  in  dem  Augc  dcs 

Menschen  und  einiger  Tiere.     Biolog.  Untersuch.  N.F.,  Bd.  6,  pag.  67  (1894). 

(iSi)  V.  Reusz,  Untersuchungen  iiber  den  Einfluss  des  Lebensalters  auf  die  Kriimniung 
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(182)  RocHON-DuviGNEAUD,  Recherches  anatomiques  sur  Tangle  dc  la  chambre 
anterieure  et  le  canal  de  Schlemm.  Archives  d'Ophthalm.,  Bd.  12,  pag.  732  und  Bd.  13, 
pag.  20,  108. 

(183)  Salzmann,  Durchschnitt  durch  das  menschliche  Auge.  Augcnarztliche  Unter- 
richtstafeln,  herausgeg.  v.  H.  Magxus,  Breslau  1899,  Heft  18. 

(184)  Die  Zonula  ciliaris  und  ihr  V'erhaltnis  zur  Umgebung.     Wien  u.  Leipzig  1900. 

(185)  tjber  die  pathologische  Anatomic  und  die  Pathologie  des  Keratokonus. 

V.  Graefes  Archiv,    Bd.  67,  pag.  i  (1907). 

(186)  Sappey,  after  Frank  Baker,  The  anatomy  of  the  eyeball,  in  Norris  amd  Oliver, 
System  of  diseases  of  the  eye,  Philadelphia  1897,  Vol.  J,  p.  113. 

(187)  Sattler,  tJber  den  feineren  Bau  der  Chorioidea  des  Menschen,  nebst  Beitragen 
zur  pathologischen  und  vergleichenden  .\natomie  der  Aderhaut.  v.  Graefes  Archiv,  Bd.  22, 
Abt.  2,  pag.  I  (1876). 

(188)  Anatomischc  und  physiologische  Beitrage  zur  Akkommodation.     Bericht 

u.  d.  19.  Versamml.  d.  ophthalm.  Gesellsch.  in  Heidelberg  1887,  pag.  3. 

(189)  Schaaff,  Der  Zentralkanal  des  Glaskorpers.  v.  Graefes  Archiv,  Bd.  67,  pag.  58; 
Bd.  71,  pag.  186  und  Bd.  75,  pag.  200  (igo8  bis  1910). 

(190)  ScHNABEL  UND  Herrenheiser,  Uber  Staphyloma  posticum,  Conus  und  INIyopie. 
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(191)  ScHOCK,  Die  Endausbreitung  des  Nervus  sympathicus  in  der  Iris.  Archiv  f.  vergl. 
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(192)  Schoen,  Die  Funktionskrankheiten  des  Auges.     Wiesbaden  1893. 

(193)  Die  Funktionskrankheiten  der  Ora  serrata  und  des  Ciliarteiles  der  Netzhaut. 

Archiv  f.  Augenheilk.,  Bd.  30,  pag.  128  (1895) — Separatabdruck.     Wiesbaden  1895. 

(194)  Schwalbe,  Lehrbuch  der  Anatomic  des  Auges.     Erlangen  1887. 

(19s)  Schweigger,  Uber  die  Ganglienzellen  und  blassen  Nerven  der  Chorioidea. 
V.  Graefes  Archiv,  Bd.  6,  Abt.  2,  pag.  320  (i860). 

(196)  ScHULTZE,  F.  E.,  Der  Ciliarmuskel  des  Menschen.  Archiv  f.  mikroskop.  Anatomic, 
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(197)  ScHULTZE,  O.,  Mikroskopische  Anatomie  der  Linse  und  des  Strahlenbandchens, 
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(198)  tJber  die  Entwicklung  und  Bedeutung  der  Ora  serrat  des  menschlichen 

Auges.    Verb,  der  physik.-medLzin.  Gesellsch.  in  Wurzburg,  N.F.,  Bd.  34,  pag.  131  (1902). 

(199)  Seefelder,  Weitere  Demonstration  embryonaler  menschlicher  Augen.  Bericht 
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(200)  ijber  die   Entwicklung   der  physiologischen   Exkavation   des  Sehnerven- 

eintrittes  beim  Menschen.    XI.  internat.  ophthalm.  Kongress,  Neapel  1909,  pag.  491. 

(201)  Untersuchungen  iiber  die  Entwicklung  der  Netzhautgefasse  des  Menschen. 

V.  Graefes  Archiv,  Bd.  70,  pag.  448  (1909). 

(202)  iiber  die  elastischen  Fasern  der  menschlichen  Cornea,  dargestellt  nach  der 

Farbemethode  von  Held.     v.  Graefes  Archiv,  Bd.  73,  pag.  1S8  (1909). 

(203)  Beitrage  zur  Histogenese  und  Histologic  der  Netzhaut.    v.  Graefes  .Archiv, 

Bd.  73,  pag.  419  viQio)- 

(204)  Das  Verhalten  der  Kammerbucht  und  ihres  Geriistwerkes  vor  der  Geburt, 

in  Graefe-Saemisch,  Handbuch   der  ges.  Augenheilk.,  2.  Aufl.,  I.  Teil,  Bd.  I,  Anhang  zu 
Kap.  II  (1910). 


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(205)  Seefelder  VXD  WoLFRtTM,  Zur  Entwicklung  der  vorderen  Kammcr  und  des 
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V.  Graefes  Archiv,  Bd.  63,  pag.  430  (1906). 

(206)  Seligiiann,  Augendiagnose  und  Kurpfuschertum.     Berlin  1910. 

(207)  SlEGRlST,  Uber  wenig  bekannte  Erkrankungsformen  des  Sehnerven.  Archiv  f. 
Augenhcilk.,  Bd.  44,  Erganzungshieft,  pag.  178  (1901). 

(20S)  SiURNOW,  Zum  Baue  der  Cliorioidea  propria  des  erwachsenen  jMenschen  (Stratum 
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(209)  Die  weisse  Augenhaut  als  Stelle  der  sensiblen  Nervenendigungen.    .Anatom. 

Anzeiger,  Bd.  18,  pag.  76  (1900). 

(210)  Spee,  Graf  F.,  tjber  den  Bau  der  Zonulafasem  und  ihre  Anordnung  im  mcnsch- 
lichen  Auge.     Anatom.  Anzeiger,  Bd.  21,  Erganzungsheft,  pag.  236  (1902). 

(211)  Steiger,  Beitrage  zur  Physiologic  und  Pathologic  der  Hornhautrefraktion.  .Archiv 
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(212)  Stilling,  Zur  Theorie  des  Glaucoms.  v.  Graefes  Archiv,  Bd.  14,  Abt.  3,  pag.  259 
(1868). 

(213)  Zur  Anatomic  des  myopischen  Augcs.     Zeitschr.  f.  Augenhcilk.,  Bd.   14, 

pag.  23  (1903). 

(214)  Str.\hl.  Zur  Entwicklung  des  menschlichen  .\uges.  Anatom.  Anzeiger,  Bd.  14, 
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(215)  Strab^,  Beitrag  zur  Kenntnis  des  Glaskorpergewebes.  v.  Graefes  Archiv,  Bd.  34, 
Abt.  3,  pag.  7  (1888). 

(216)  Szili,  A.,  Beitrage  zur  Kenntnis  der  Anatomic  und  Entwicklungsgeschichte  der 
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(217)  V.  SzQy,  A.,  liber  das  Entstehen  eines  fibrOaren  Sttitzgewebes  im  Embrj-o  und 
dessen  Verhaltnis  zur  Glaskorperfrage.    Anatom.  Hefte,  Heft  107  (1908). 

(218)  Takayasu,  Beitrage  zur  pathologischcn  Anatomic  des  Arcus  senilis.  Archiv  f. 
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(219)  Tandler,  titer  einen  menschlichen  Embryo  vom  38  Tage.  Anatom.  Anzeiger, 
Bd.  31,  pag.  49  (  =  Keibel,  Normentafeln  Nr.  42). 

(220)  Tartuferi,  iiber  das  elastische  Homhautgewebe  und  iiber  eine  besondcre  Mctal- 
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(221)  Terrien,  Recherches  sur  la  structure  de  la  retine  ciliare  et  I'origine  des  fibres  de 
la  zonule  de  Zinn.     Paris  1898. 

(222)  Topolanski,  tiber  den  Bau  der  Zonula  und  Umgebung,  nebst  Bemerkungen  iiber 
das  albinotische  Auge.    v.  Graefes  Archiv  Bd.  37,  Abt.  i,  pag.  28  (1891). 

(223)  Linsenranderhebungen.     Klin.  Monatsbl.  f.  .•\ugenheilk.  (1892),  30.  Jahrg., 

pag.  89. 

(224)  Torj;.4TOLa,  Sulla  membrane  limitante  interna  deUa  retina  nei  vertebrati.  Anatom. 
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(225)  Trantas,  Moyen  d'cxplorer  par  I'ophthalmoscopc  et  par  translucidite  la  parti 
anterieure  du  fond  oculairc,  le  ccrcle  ciliare  y  compris.    Archives  d'Ophthalm.    Juin  igoo. 

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(226)  Treacher  Collins,  The  glands  of  the  cUiary  body  in  the  human  eye.  Tran- 
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(227)  TSCHERNING,  Optique  physiologique.     Paris  1898. 

(228)  Hermann  v.  Helmholtz  und  die  .Akkommodationstheoric,  ubersetzt  von 

Thorey.    Leipzig  1910. 

(229)  Ulbrich,  Klinische  Bcobachtungen  iiber  die  Drukverhaltnisse  in  der  vorderen 
und  hinteren  Kammer.    Archiv  f.  Augenhcilk.,  Bd.  60,  pag.  283  (1908). 


226  ANATOMY  AND  HISTOLOGY  OF  THE  HUMAN  EYEBALL 

(230)  UsHKR,  A  note  on  the  chorioid  at  the  mucuhir  region.     Transactions  of  the  Ophth. 
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(231)  Versari,  La  morfogcnesi  dei  vasi  sanguigni  della  retina  umana.     Ricerche  fatte 
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Anzeiger,  Bd.  35,  pag.  105  (1909). 

(232)  ViRCHOW,  H.,  tjber  die  Form  der  Fallen  des  Corpus  ciliare  bei  Saugctieren.    Mor- 
pholog.  Jahrbucher,  Bd.  11,  pag.  437 — Nagels  Jahresber.  f.  1885,  pag.  10. 

(233) Facher,  Zapfen,  Leiste,  Polster,  Gefasse  im  Glaskorperraum  von  Wirbeltieren, 

in  Merkel  und  Bonnet,  Ergebnisse  d.  Anatomie  und  Entwicklungsgeschichte,  Bd.  10  (1900). 

(234)  Mikroskopische  Anatomie  der  ausseren  Augenhaut  und  des  Lidapparates,  in 

Graefe-Saemisch,  Handbuch  d.  ges.  Augenheilk.,  2.  Aufl.,  I.  Teil.  Bd.  L  Kap.  II  (1908). 

(235)  Weiss,  L.,  Uber  das  Wachstum  des  menschlichen  Auges.    Anatom.  Hefte,  Bd.  8, 
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(236)  Wessely,  liber  Versuche  am  wachsenden  Auge.     Miinth.  mediz.  Wochenschr. 
1909,  Nr.  44. 

(237)  WiDMARK,  Uber  den  Musculus  dilatator  pupiUae  des  Menschen.    Mitteil.  a.  d. 
Augenklinik  d.  Carol,  med.-chir.  Inst,  in  Stockholm,  Heft  3,  pag.  25  (1901). 

(238)  WiEGER,  Uber  den  Canalis  Petiti  und  ein  Ligamentum  hyaloideo-capsulare.     Inaug. 
Dissert.  Strassburg  18S3. 

(239)  WoLFRUM,  Zur  Entwicklung  und  normalen  Struktur  des  Glaskorpers.    v.  Graefes 
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(240)  Beitrage  zur  Anatomie  und  Histologic  der  Aderhaut  beim  Menschen  und 

bei  den  hoheren  Wirbeltieren.     v.  Graefes  Archiv,  Bd.  67,  pag.  307  (1908). 

(241)  Untersuchungen  iiber  die  Macula  lutea  der  hoheren  Saugetiere.     Bericht  ii. 

d.  35.  Vers.  d.  ophthalm.  Gesellsch.  in  Hiedelberg  1908,  pag.  206. 

(242)  tJber  den  Ursprung  und  Ansatz  der  Zonulafasern  im  menschlichen  Auge. 

V.  Graefes  Archiv,  Bd.  69,  pag.  145  (1908). 

(243)  Zeeman,  Uber  die  Form  der  hinteren  Linsenflache.    Klin.  Monatsbl.  f.  Augenheilk., 
46.  Jahrg.,  I,  pag.  83  (1908). 

(244)  Linsenmessungen  und  Emmetrepisation.     v.  Graefes  Archiv,  Bd.  78,  pag.  93 

(1911). 


EXPLANATION  OF  PLATE  FIGURES 

PLATE  I 

The  iris  and  the  ciliary  body  with  their  environment,  meridional  section;  magnification  30. 

Mr  tendon  of  the  musculus  rectus  medialis,  Es  episcleral  tissue,  Co  conjunctiva  sclerae,  CS  corneo- 
scleral border,  b  border  of  Bowman's  membrane,  d  Descemet's  membrane,  ^i  ciliary,  ki  pupillary  crypts, 
/contraction  furrows;  iz  innermost,  cZ  circular,  vZ  anterior,  qZ  middle,  hZ  posterior  zonula  fibers. 

hG  posterior  border  layer  of  the  vitreous,  gB  vitreous  base,  Z  zonular  cleft,  dG  anterior  border 
layer,  Lhc  ligamentimi  hyaloideo-capsulare,  Gk  vitreous  nucleus. 

PLATE  II 

1.  Anterior  half  of  the  eye  seen  from  within;  magnification  3. 

N  nasal,  T  temporal  side,  Os  ora  serrata  retinae.  Or  orbiculus  ciliaris.  Cor  corona  ciliaris,  CI  cir- 
cumlental  space,  L  posterior  surface  of  the  lens  with  the  lens  star. 

2.  Emissarium  of  the  lateral  long  posterior  cUiary  artery;  cross-section,  magnification  87* 
A  arter)-,  M  ciliary  nerve. 

3.  Emissarium  of  the  superior  temporal  vortex  vein ;  longitudinal  section ;  magnification  23. 
Os  tendon  of  the  musculus  obliquus  superior,  Te  extension  of  Tenon's  space  at  the  exit  of  the  vein, 

V  the  lumen  of  the  vortex  vein  itself,  5  sclera,  Pch  perichorioidal  space,  Ch  chorioidea,  P  pigment 
epithelium. 

4.  Emissarium  of  a  vortex  vein;  cross-section  somewhat  behind  the  center;  magnifica- 
tion 87. 

5.  Details  of  the  sclera,  middle  reaches,  region  of  the  ciliary  body,  meridional  section; 
magnification  285. 

A  small  artery,  surrounded  by  loose  connective  tissue,  c  capillary,  q  cross-sectioned,  so-called 
equatorial  bundle,  /  longitudinal  section,  meridional  bundle. 

6.  Anterior  Ia5'ers  of  the  cornea,  meridional  section;  magnification  400. 

Ep  epithelium,  0  superficial,  b  basal  cells,  B  Bowman's  membrane  with  a  nerve  pore,  C  stroma 
corneae,  s  spaces  therein. 

7.  Elementar>'  lameDae  of  the  stroma  corneae;  spontaneous  dehiscence  in  meridional 
section;    magnification  450. 

z  a  fixed  corneal  cell,  q  cross-sectioned,  /  longitudinally  sectioned,  elementary  lamellae. 

8.  Fixed  corneal  cells  seen  from  the  surface,  Held's  stain;  magnification  375. 
w  a  wandering  cell,  /  elastic  fibers. 

9.  Posterior  layers  of  the  cornea,  meridional  section;   magnification  475. 

C  stroma  corneae,  i  spaces  with  fixed  cells,  D  Descemet's  membrane,  en  endothelium. 

10.  Details  of  the  trabeculum  of  the  iris  angle,  inner  layers,  meridional  section;  magni- 
fication 510. 

i  trabecula  of  the  uveal  framework,  the  upper  one  cut  longitudinally,  the  lower,  cut  across;  the 
remaining  trabeculae  belong  to  the  scleral  framework,  b  collagenous  connective  tissue,/ elastic  fibers, 
g  glass  membrane,  e  endothelium. 

PLATE  III 

I.  The  scleral  furrow  with  its  environment,  meridional  section;   magnification  96. 

C  cornea,  D  border  of  Descemet's  membrane,  vG  the  anterior  border  ring,  T  deep  root  of  the  scleral 
framework,  i  uveal  framework,  Sch  lumina  of  the  Schlemm's  canals,  V  efferent  veins  of  these  canals. 
Sin  scleral  roll,  Pch  beginning  of  the  perichorioidal  space,  Mc  musculus  ciliaris,  /  iris  root. 

227 


22S  ANATOMY  AND  HISTOLOGY  OF  THE  HUMAN  EYEBALL 

2.  Anterior  border  of  the  framework  with  its  roots,  surface  view,  combination  picture 
from  teased  preparations;   magnification  112. 

D  smooth  portion  of  Descemet's  membrane  and  endothelium  with  plain  cell  borders,  ir  beginning  of 
the  wart  zone  (the  warts  appear  as  darker  flecks),  vG  anterior  border  ring,  d  border  of  Descemet's  mem- 
brane, T  deep  root  of  the  framework,  i  uveal  framework,  Tr  inner  reaches  of  the  scleral  framework  w'ilh 
diagonal  spaces,  Tr'  outer  reaches  of  the  framework  with  more  roundish  spaces. 

3.  Uveal  framework,  surface  view,  teased  preparation;  magnification  112. 

T  part  of  the  scleral  framework,  e  endothelial  cells  undergoing  detachment,  /  transition  Into  the 
tissue  of  the  iris  root. 

4.  A  long  posterior  ciliary  artery  with  its  accompanying  nerves  in  the  perichorioidal  space, 
equatorial  section;  magnification  75. 

Ch  chorioidea,  S  sclera,  A  artery,  m  connective  tissue  with  tiny  bundles  of  smooth  muscle-fibers, 
iV  the  larger,  N2  the  smaller,  accompanying  nerve. 

5.  A  vortex  vein,  surface  view;  magnification  5. 

6.  Details  of  the  inner  reaches  of  the  vessel  layer  of  the  chorioidea,  teased  preparation; 
magnification  285. 

V  part  of  a  small  vein,  e  its  endothelium,  p  its  perivascular  sheath,  a  its  adventitia;  A  small  arter>-, 
m  its  muscle-fibers,  b  connective  tissue  of  the  stroma;  ch  chromatophores,  w  wandering  cells,  n  nerve- 
fibers,  g  ganglion  cells. 

7.  Details  of  the  innermost  layers  of  the  chorioidea,  terrace-form  torn  border,  teased 
preparation,  Held's  stain;  magnification  430. 

cu  cuticular  lamella,  el  elastic  lamella  of  the  glass  membrane  of  the  chorioidea,  k  lumina  of  the 
capillaries,  r  red  blood  corpuscles  therein,  /  a  leucocyte,  e  nuclei  of  the  endothelium  of  the  capillar>'  wall, 
interstices  of  the  capillary  layer;  the  dark  points  along  the  contours  of  the  capillaries  are  the  optical 
cross-sections  of  the  elastic  fibrillae,/  subcapillary  fibrillar  net,  sc  subcapillary  cells. 

8.  Pigment  epithelium  of  the  chorioidea,  middle-zone,  surface  view;   magnification  472. 

9.  Individual  pigment  granules  of  the  pigment  epithelium  of  the  chorioidea;  magnification 
1,500. 

10.  Pigment  epithelium  of  the  chorioidea  from  the  region  of  the  fovea  centralis,  surface 
view;  magnification  472. 

11.  Pigment  epithelium  of  the  chorioidea  in  the  neighborhood  of  the  ora  serrata  retinae, 
surface  view;  magnification  472. 

PLATE   IV 

1.  Ending  of  Descemet's  membrane,  and  the  anterior  border  of  the  trabeculum  of  a 
53-year-old  man,  meridional  section;    magnification  300. 

C  stroma  corneae,  T  deep  root  of  the  trabeculum,  D  Descemet's  membrane,  w  its  warts,  d  its  border, 
vG  anterior  border  ring.  En  endothelium  of  the  cornea,  i  a  fiber  of  the  uveal  trabeculum. 

2.  Elements  of  the  suprachorioidea,  teased  preparation,  staining  with  Mallor>''s  phospho- 
molybdic-acid  hematoxylin;    magnification  300. 

ch  chromatophores,/  elastic  fibers,  e  endothelial  nuclei,  n  branch  of  a  nerve  with  a  ganglion  cell. 

3.  Chorioidea  and  retina  nasal  to  the  optic-nerve  entrance,  meridional  section;  magni- 
fication 286. 

5  sclera,  Su  suprachorioidea,  Gj  vessel  layer  of  the  chorioidea,  A  artery,  V  small  vein,  emptying 
at  right  angles  into  the  choriocapillaris  C,  Lv  lamella  vitrea  chorioideae,  P  pigment  epithelium,  Pf  its 
pigment  processes,  SZ  rod-and-cone  layer,  a  outer  member,  i  inner  member,  Le  membrana  limitans 
externa,  aK  outer  nuclear  layer,  5  rod  nucleus,  z  cone  nucleus,  ap  outer  plexiform  layer,  /  the  distal 
fibrous  division,  r  the  proximal  reticular  division,  .r  border  between  the  neuroepithelium  and  cerebral 
layer,  iK  inner  nuclear  layer,  c  capillaries,  ip  inner  plexiform  layer.  G  ganglion-cell  layer,  X  ner\-e-fiber 
layer,  gl  glial  nuclei,  Li  membrana  limitans  interna,  M  basal  cones  of  Mueller's  supporting  fibers. 

4.  Entrance  of  the  optic  nerve  (papilla  nervi  optici),  horizontal  section;  magnification  31. 
T  temporal.  A'  nasal  side,  D  dural  sheath,  ^r  arachnoidal  sheath,  P  pial  sheath,  I  intervaginal  space. 

Ma  medullatcd  portion  of  the  optic  nerve,  cB  central  connective-tissue  strand,  V  vena  centralis  retinae, 


EXPLANATION  OF  PLATE  FIGURES  229 

A  arleria  centralis  retinae,  Lc  lamina  cribrosa  (1.  scleralis).  Lch  lamina  chorioidalis,  Ks  nuclear  column, 
Me  central  connective  tissue  meniscus.  £.v  physiologic  exxavation,  5  sclera,  Pch  perichorioidea,  Ch 
chorioidea,  gr  border  tissue,  R  retina,  int  intermediary  tissue. 

5.  The  trabecular  work  of  the  intervaginal  space,  longitudinal  section;  magnification  288. 
Ar  arachnoidal  sheath,  aE  its  outer  endothelium,  iE  its  inner  endothelium,  Vb  trabecula  of  union 

between  the  dural  and  pial  sheaths  in  cross-section,  surrounded  by  an  invagination  of  the  arachnoidal 
sheath,  sb  primitive  subarachnoidal  trabeculum,  SB  large  subarachnoidal  trabeculum,  P  pial  sheath. 

6.  Arachnoidal  sheath,  surface-section;    magnification  410. 

a  outer  reticular  connective-tissue  layer.  /  inner  connective-tissue  layer  consisting  of  separated 
bundles,  E  endothelium  nuclei. 

7.  Choriocapillaris  in  the  neighborhood  of  the  posterior  pole,  natural  injection  through 
hypostasis,  surface  view,  teased  preparation;    magnification  65. 

.T  the  places  where  the  smallest  arteries  and  veins  unite  with  the  capillary  layer. 

8.  A  small  bundle  of  smooth  muscle-fibers  in  the  suprachorioidea,  surface  view,  teased 
preparation,  staining  with  MalIorj''s  phospho-molybdic-acid  hemato.x>'lin ;  magnification  2C0. 

ch  chromatophores,  e  endothelial  nuclei.  11  non-meduUated  nerve-fiber,  m  smooth  muscle-fibers 
supported  by  numerous  elastic  fibers  coursing  out  in  pencil-formed  brushes  (/). 

9.  Posterior  border  of  the  ciliary  muscle,  surface  view,  teased  preparation;  magnification  32. 
M  meridional  portion  of  the  ciliary  muscle,  st  muscle-stars,  N  branches  of  the  ciliary  nerves. 

10.  A  sector  of  the  inner  surface  of  the  ciliary  body  and  of  the  circumlental  space;  magni- 
fication, 15. 

Os  ora  serrata  retinae  with  2  teeth  (0)  each  of  which  continue  into  a  stria  ciliaris  {St),  Or  orbiculus 
ciliaris,  Pc  ciliary  processes,  w  the  small  warts  between,  CI  circumlental  space  with  the  posterior  zonula 
fibers,  L  border  of  the  lens. 

PLATE  V 

1.  Choriocapillaris  in  the  neighborhood  of  the  ora  serrata  retinae,  natural  injection,  surface 
view,  teased  preparation;   magnification  56. 

The  smaller  lighter  vessels  belong  to  the  vessel  layer  of  the  chorioidea,  .4  a  recurrent  artery. 

2.  The  rod  and  cone  mosaic  in  the  extrafoveal  territor>'  of  the  retina,  surface-section  at 
the  level  of  the  inner  members;  staining  with  Mallory's  hematoxyhn;   magnification  400. 

The  larger  disks  are  the  cross-sections  of  cones,  the  smaller,  of  rods. 

3.  Details  from  the  center  of  a  very  flat  fovea;  rod-free  area,  temporal  half,  horizontal 
section;   magnification  300. 

Li  membrana  limitans  interna  retinae,  ;A'  rudiment  of  the  cerebral  layer,  Hf  Henle's  outer  fiber 
layer,  (7 A'  outer  nuclear  layer,  Le  membrana  limitans  externa,  Z  cone  layer, /Z  the  thin  foveal  cones. 

4.  The  nasal  half  of  a  small  fovea  centralis,  horizontal  section;  magnification  47. 

F  middle  of  the  fovea  (so-called  foveola),  cl  clivus,  w  wall  about  the  fovea,  g  larger  blood-vessel  in 
the  ganglion-cell  layer,  Li  membrana  limitans  interna,  N  nerve-fiber  layer,  G  ganglion-cell  layer,  ip  inner 
plexiform  layer,  iK  inner  nuclear  layer,  Hf  Henle's  outer  fiber  layer,  aK  outer  nuclear  layer,  5:  rod-and- 
cone  layer,  P  pigment  epithelium,  Ch  chorioidea,  S  sclera. 

5.  Border  portion  of  the  retina  with  cystic  degeneration,  surface  view;   magnification  55. 
Pc  pigment  epithelium  of  the  ciliary  body,  R  retina,  H  cystic  spaces,  for  the  most  part  confluent 

into  dendritic  figures. 

6.  Border  portions  of  the  retina  with  cystic  degeneration,  meridional  section;  magnifica- 
tion 47. 

Ce  ciliary  epithelium,  Os  ora  serrata,  R  retina,  Ch  chorioidea;  the  place  indicated  by  X  is  drawn 
under  seven  times  magnification. 

7.  Cystoid  degeneration  of  the  retina  (from  the  place  marked  x  in  6);   magnification  315. 
Li  membrana  limitans  interna,  G  rudiment  of  the  nerve-fiber  and  ganglion-cell  layer,  ij  retinal 

vessel,  ip  inner  plexiform  layer.  iK  inner  nuclear  layer,  ap  outer  plexiform  layer.  aK  outer  nuclear  layer, 
H  cystic  spaces,  Le  membrana  limitans  externa,  SZ  rod-and-cone  layer. 

8.  Cells  on  the  surface  of  the  base  of  the  vitreous;  staining  with  ]Mallor>''s  hemato.xylin; 
magnification  255. 


230         ANATOMY  AND  HISTOLOGY  OF   THE  HUMAN  EYEBALL 

PLATE  VI 

1.  Middle  layer  of  the  lamina  crihrosa,  surface-section,  staining  with  Van  Giesen;  magni- 
fication 255. 

B  the  connective-tissue  glial  trabeculae,  g  blood-vessels  therein,  N  nerve-fiber  bundle. 

2.  Cross-section  of  the  optic  nerve  at  the  posterior  end  of  the  central  connective-tissue 
strand,  staining  by  Van  Giesen;  magnification  22. 

D  dural  sheath,  sd  subdural  space,  ar  arachnoidal  sheath,  sar  subarachnoidal  space,  P  pial  sheath, 
Glm  peripheral  glial  mantle,  V  vena  centralis  retinae,  A  arteria  centraHs  retinae. 

3.  Details  from  2 ;  magnification  312. 

s  connective-tissue  septa,  g  blood-vessels  therein,  Gl  glial  extension  of  the  septa,  N  nerve-fiber 
bundle,  Ly  spaces  arising  from  shrinking. 

4.  A  piece  of  the  medullated  portion  of  the  optic  nerve  in  longitudinal  section;  magnifica- 
tion go. 

6'i  surface  view  of  the  septa,  S,  longitudinal  section  of  the  septa,  Gl  glial  continuation  of  the  septa, 
N  nerve-fiber  bundle. 

5.  Details  of  the  cross-section  of  a  nerve-fiber  in  the  medullated  portion  of  the  optic  nerve, 
Mallory's  stain;   magnification  950. 

n  nerve-fibers,  g!  glial  fibers,  =  a  glial  cell. 

6.  The  superficial  portions  of  the  optic -ner\'c  trunk,  cross-section,  Weigert's  stain  for 
medullary  sheaths;   magnification  63. 

D  dural  sheath,  sd  subdural  space,  Ar  arachnoidal  sheath,  sar  subarachnoidal  space,  P  pial 
sheath,  Glm  peripheral  glial  mantle,  N  nerve-fiber  bundle,  S  septa,  Q  artefact  arising  through  squeezing. 

PLATE  VII 

1.  The  fundus  of  a  13-year-old  girl  with  a  brown  iris,  seen  with  the  ophthalmoscope; 
papilla,  retinal  blood-vessels,  macular  and  foveal  reflex. 

2.  Corona  ciliaris  (posterior  half)  and  neighboring  portions,  transverse  section;  magni- 
fication 47. 

Gk  nucleus  of  the  vitreous,  vG  anterior  border  layer,  Lc  ligaments-cordiformes,  Z  cross-sections  of 
the  zonula  fibers  in  the  ciliary  valleys,  iz  innermost  zonula  fibers,  Li  membrana  limitans  interna  ciliarisi 
CE  ciUarj'  epithelium,  P  pigment  epithelium,  Gf  vessel  layer  of  the  ciliary  body,  Pc  ciliary  processes, 
Mc  ciliary  muscle  (innermost  layers  of  the  radial  portions). 

3.  Reticulum  of  H.  Mueller  in  the  anterior  part  of  the  orbicttlus  cUiaris,  surface  view^ 
teased  preparation,  stained  by  Mallory's  hemato.vylin;   magnification  285. 

The  arrow  below  indicates  the  direction  of  the  meridian  and  points  forward. 

4.  Orbiculus  ciliaris  in  the  neighborhood  of  the  ora  serrata  retinae,  transverse  section, 
bleached;  magnification  320. 

Gb  vitreous  base,  Li  membrana  limitans  interna  ciliaris  (?),  CE  ciliary  epitheUum,  P  pigment 
epithelium,  /  ridges  of  the  reticulum  of  H.  Mueller  (large  meshes),  Cii  cuticular  lamella,  iB  interlamellar 
connective  tissue,  el  elastic  lamella,  Gf  vessel  layer  of  the  ciliary  body. 

5.  Orbiculus  ciliaris  in  the  neighborhood  of  the  corona  ciliaris,  transverse  section,  bleached; 
magnification  380. 

tG  anterior  border  layer  of  the  vitreous,  Z  cross-sections  of  the  zonula  fibers  in  the  orbicular  space, 
Li  membrana  limitans  interna  ciliaris,  /  its  ridges,  inclosing  in  part  finest  zonula  fibers,  CE  ciliary 
epithelium,  P  pigment  epithelium,  Cu  cuticular  lamella  (small  meshes  of  the  reticulum  of  H.  Mueller), 
iB  interlamellar  connective  tissue,  el  elastic  lamella,  Gf  vessel  layer  of  the  cihary  body. 

6.  Ciliary  border  of  the  sphincter  pupillae  with  the  spoke  bundles,  surface-section; 
magnification  348. 

Spit  sphincter  pupillae,  Sp  spoke  bundle,  A'  clump  cells,  b  collagenous  intermediary  substance  of 
the  iris  stroma. 

7.  Senile  warts  of  the  glass  membrane  of  the  chorioidea,  magnification  390. 


EXPLANATION  OF  PLATE  FIGURES  231 

PLATE  VIII 

1.  Drawing  of  the  anterior  iris  surface  (yellowish-gray  iris,  flecked  here  and  there  by 
rust-brown)  unstained,  reflected  light;  magnification  9. 

Pz  pupillary  zone,  Cz  ciliary  zone,  Ps  pigment  seam  of  the  pupil-border,  kj  pupillary  crypts,  Z 
angular  line  (smaller  circle),  N  brown  fleck,/  contraction  furrow,  kt  ciliary  (peripheral)  crypts  in  the 
border  zone. 

2.  Relief  of  a  posterior  iris  surface,  under  reflected  light;   magnification  9. 

rF  radial  furrows  of  the  pupillarj'  zone,  SF  structural  furrows,  cF  circular  furrows,  C  anterior 
border  of  the  corona  ciliaris. 

3.  Pupillary  zone  of  the  iris,  meridional  section;  magnification  60. 

uG  anterior  border  layer  (with  endothelium),  G  vessel  layer,  Di  dilatator  pupillae,  P  pigment  epi- 
thelium, Sph  sphincter  pupillae,  Sp  spoke  bundle,  e  racUations  of  the  dilatator  into  the  sphincter  from 
behind,  Ps  pigment  seam  of  the  pupillary  border,  K  clump  cells. 

4.  The  markings  upon  the  anterior  surface  of  the  iris  arising  after  treatment  with  silver 
nitrate;  magnification,  636. 

5.  Plexus  of  the  chromatophores  in  the  anterior  border  layer  of  a  brown  iris — out  of  the 
outer  half  of  the  ciliary  zone,  surface  view,  teased  preparation;  magnification  375. 

6.  Ciliary  border  of  the  dilatator  pupillae,  surface  view;  magnification  28. 
SF  structural  furrows. 

7.  Elements  of  the  dilatator  pupillae  in  the  middle  of  the  ciliary  zone,  surface  view,  teased 
preparation;    magnification  375. 

8.  Ectodermal  layers  of  the  posterior  iris  surface,  meridional  section,  bleached;  magni- 
fication 260. 

Sir  stroma  of  the  vessel  layer  of  the  iris,  ItG  posterior  border  lamella,  sp  la>er  of  the  pigmented 
spindle  cells,  P  pigment  epithelium. 

9.  Ectodermal  layers  of  the  posterior  iris  surface,  transverse  section,  bleached;  magni- 
fication 330. 

Annotations  as  in  8. 

10.  Posterior  layers  of  the  pupillary  zone  of  the  iris,  transverse  section,  bleached;  magni- 
fication 300. 

Str  stroma  of  the  vessel  layer,  Sph  sphincter  pupillae,  A'  clump  cells,  Di  pupillary  border  of  the 
dilatator  pupillae  constructed  out  of  transition  forms  between  tj-pical  dilatator  elements  and  epithelial 
cells;  in  the  center  is  a  union  with  the  sphincter,  P  pigment  epitheUum. 

11.  Inner  layers  at  the  anterior  declivity  of  a  ciliary  process  (transition  of  the  cih'ary 
epithelium  into  the  pigment  epithelium  of  the  iris),  meridional  section;  magnification  375. 

Li  membrana  limitans  interna  ciliaris,  CE  cihary  epithelium,  P  pigment  epithelium  of  the  ciliary 
body,  Cu  cuticular  lamella,  Of  vessel  layer  of  the  ciharj'  body  (processes). 

12.  Narrow  iris  (iris  breadth  3  mm,  pupil  width  5  mm);  magnification  20. 

13.  Broad  iris  (iris  breadth  5  mm,  pupil  width  2.5  mm);   magnification  20. 

14.  Radiations  of  zonula  fibers  into  the  anterior  border  layer  of  the  vitreous;  a  detail 
out  of  a  surface  preparation  of  the  anterior  border  layer,  stained  with  JNIallory's  hematoxylin; 
magm'fication  67. 

vG  anterior  border  layer,  finely  folded,  m  a  meridional  zonula  fiber  radiating  into  the  border  layer 
at  two  places,  k  a  short  bundle  of  zonula  fibers  coming  from  the  corona  ciliaris,  radiating  in  its  entirety 
into  the  border  layer,  farther  above  a  smaller  similar  bundle,  cZ  circular  zonula  fibers. 

PLATE  IX 

1.  Equatorial  zone  of  the  lens  capsule  with  the  insertion  of  the  zonula  fibers,  surface  view, 
teased  preparation;  magnification  37. 

vZ  anterior  zonula  fibers,  Zl  meridional  striations  of  the  zonular  lamella,  qZ  middle  (equatorial) 
zonula  fibers,  EG  location  of  the  epithelial  border,  liZ  posterior  zonula  fibers. 

2.  Lens  epithelium  treated  with  silver  nitrate,  surface  view;  magnification  540. 


232  ANATOMY  AND  HISTOLOGY  OF  THE  HUMAN  EYEBALL 

3.  The  lens  vortex  of  a  59-year-old  man,  meridional  section;   magnification  m. 

Zl  zonular  lamella  with  ligamcntum  hyaloideo-capsularis,  K  lens  capsule,  E  lens  epithelium,  Eg 
epithelial  border,  /  lens  substance. 

4.  Superficial  layers  of  the  lens  of  a  2-vvceks-old  child,  equatorial  section;  magnification 
308. 

K  lens  capsule,  E  lens  epithelium,  L  lens  substance  (Rabl's  lamellae). 

5.  Deeper  layers  of  the  same  preparation  (1.3  mm  below  the  surface);  irregular  arrange- 
ment and  form  of  the  lens  fibers. 

6.  Optic  grooves  in  an  open  medullary  canal,  embryo  of  2.6  mm  gr.  1.;   magnification  21. 
(Copy  from  Keibel,  normal  plates  (116)  No.  6,  p.  24,  Te.xt  Fig.  6  a.) 

E  ectoderm,  W  wall  of  the  medullary  canal,  A  the  optic  grooves,  M  mesoderm. 

7.  Transition  of  the  (primary)  optic  vesicle  into  the  optic  cup,  beginning  of  the  lens 
invagination,  embryo  of  5.  2  mm  gr.  1.,  section  in  the  direction  of  the  optic  cleft;  magnification 
66. 

(Collection  of  the  I.  Anatomic  Institute  of  Vienna.) 

E  ectoderm,  L  lens  primordiiun,  M  mesoderm,  IV  wall  of  the  medullary  canal  (forebrain),  V  lumen 
of  the  same,  S  lumen  of  the  pedicle  of  the  optic  vesicle,  .1  lumen  of  the  (primary)  optic  vesicle,  a  outer 
leaf  of  the  optic  cup,  i  inner  leaf  of  the  eye-cup. 

8.  Completely  formed  optic  cup,  optic  cleft  for  the  most  part  closed,  lens  vesicle  con- 
stricted off,  beginning  of  the  formation  of  the  definite  cornea,  embryo  of  9.  75  mm  gr.  1.,  section 
in  the  direction  of  the  optic  cleft;  magnification  66. 

(Collection  of  the  I.  Anatomic  Institute  of  Vienna;  Keibel,  normal  plates  (116)  No.  42,  Tandler, 
207.) 

E  ectoderm,  L  lens  vesicle,  M  mesoderm,  Mf  mesodermal  processes  in  the  cavity  of  the  optic  cup, 
H  primitive  cornea,  G  primitive  vitreous,  W  wall,  v  lumen  of  the  forebrain,  ^  lumen  of  the  pedicle  of  the 
optic  vesicle,  A  lumen,  a  outer,  i  inner  wall  of  the  optic  cup. 

9.  Anterior  segment  of  the  eyeball,  embryo  of  28.  5  mm  gr.  1.;  magnification  40. 
(Collection  of  the  I.  Anatomic  Institute  of  Vienna.) 

E  ectoderm,  Fo  primordium  of  the  fornix  conjunctivae,  //  primordium  of  the  cornea  stroma,  IP 
lamina  irido-pupillaris,  isthmus,  L  lens,  Tv  tunica  vasculosa  lentis,  G  vitreous  with  its  vessels,  R  retina 
(inner  leaf  of  the  optic  cup),  P  pigment  epithelium  (outer  leaf  of  the  optic  cup),  5  sclera  (the  primordium 
of  the  chorioidea  is  not  plainly  visible  because  of  the  low  magnification).  The  clefts  between  H  and  IP, 
as  well  as  between  R  and  P,  are  artefacts. 

10.  Primordium  of  the  iris  and  of  the  ciliary  body,  fetus  of  18  cm  1.,  meridional  section; 
magnification  35. 

E  epithelium  of  the  cornea,  C  corneal  stroma,  D  endothelium  of  the  cornea,  Li  limbus,  5  sclera,  Vk 
anterior  chamber,  Pm  pupillary  membrane,  I  iris  primordium,  Sph  sphincter  primordium,  Pc  primor- 
dium of  the  corona  ciliaris,  Mc  primordium  of  the  ciliary  muscle,  P  pigment  epithelimn,  R  retina,  G 
vitreous,  Z  zonula  primordium,  Tv  tunica  vasculosa  lentis,  L  lens. 

11.  The  border  of  the  retina  (ora  serrata)  of  the  newborn,  meridional  section;  magnifica- 
tion 116. 

R  retina,  F  apex  of  the  so-called  Lange's  fold,  CE  ciliary  epithelium,  P  pigment  epithelium,  U  uveal 
tract,  S  sclera. 


f      8. 


'# 


PhotocoIIolype  Max  Jaffe,  Vienna. 


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Pliolocoilolype  Max  J.iffe,  Vi^-nna. 


Salzmann. 


pi.  IV. 


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Pliolocollotype  Max  Jiilte,  Vienna 


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Phoi>jcollotype  Max  Jalfe,  Vienna. 


Salzmann, 


PI.  VI. 


Hhotocollotype  Max  Jalfe,   VIcnnii, 


PI.  VII. 


I'hulucollotypc  M.ix  JaKt,  Vicniin 


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Photocollotype  Max  Jaffe,  Vienna. 


Salzmann, 


Tv        •  R  PS 

Photocollotype  Max  Jaffe,  Vienn 


COLUMBIA  UNIVERSITY  LIBRARIES 


This  book  is  due  on  the  date  indicated  below,  oi-  at  the 
expiration  of  a  definite  period  after  the  date  of  borrowing, 
as  provided  by  the  library  rules  or  by  special  arrangement 
with  the  Librarian  in  charge. 


DATE  BORROWED 

DATE  DUE 

DATE  BORROWED 

DATE  DUE 

1 

i      ' 
1 

L    '■' 

Cza    {12641    BOM 

Salzraann 


QM511 

Sa31 

1912 


thI\^ra:\7^,-^^^i3tolog,  or 


5a,   3/ 
/f/2_ 


Annex 


I'l'i!'!!!}'! 


:|:if!'!i;!iiil!  iliiiii 


